5322
DRAFT STANDARD
Internet Message Format
Authors: P. Resnick
Date: October 2008
Working Group: NON WORKING GROUP
Stream: IETF
Abstract
This document specifies the Internet Message Format (IMF), a syntax for text messages that are sent between computer users, within the framework of "electronic mail" messages. This specification is a revision of Request For Comments (RFC) 2822, which itself superseded Request For Comments (RFC) 822, "Standard for the Format of ARPA Internet Text Messages", updating it to reflect current practice and incorporating incremental changes that were specified in other RFCs. [STANDARDS-TRACK]
RFC 5322
DRAFT STANDARD
Updated by: 6854 Errata Exist
Network Working Group P. Resnick, Ed.
Request for Comments: 5322 Qualcomm Incorporated
Obsoletes: <a href="./rfc2822">2822</a> October 2008
Updates: <a href="./rfc4021">4021</a>
Category: Standards Track
<span class="h1">Internet Message Format</span>
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
This document specifies the Internet Message Format (IMF), a syntax
for text messages that are sent between computer users, within the
framework of "electronic mail" messages. This specification is a
revision of Request For Comments (RFC) <a href="./rfc2822">2822</a>, which itself superseded
Request For Comments (RFC) <a href="./rfc822">822</a>, "Standard for the Format of ARPA
Internet Text Messages", updating it to reflect current practice and
incorporating incremental changes that were specified in other RFCs.
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Table of Contents
<a href="#section-1">1</a>. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-4">4</a>
<a href="#section-1.1">1.1</a>. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-4">4</a>
<a href="#section-1.2">1.2</a>. Notational Conventions . . . . . . . . . . . . . . . . . . <a href="#page-5">5</a>
<a href="#section-1.2.1">1.2.1</a>. Requirements Notation . . . . . . . . . . . . . . . . <a href="#page-5">5</a>
<a href="#section-1.2.2">1.2.2</a>. Syntactic Notation . . . . . . . . . . . . . . . . . . <a href="#page-5">5</a>
<a href="#section-1.2.3">1.2.3</a>. Structure of This Document . . . . . . . . . . . . . . <a href="#page-5">5</a>
<a href="#section-2">2</a>. Lexical Analysis of Messages . . . . . . . . . . . . . . . . . <a href="#page-6">6</a>
<a href="#section-2.1">2.1</a>. General Description . . . . . . . . . . . . . . . . . . . <a href="#page-6">6</a>
<a href="#section-2.1.1">2.1.1</a>. Line Length Limits . . . . . . . . . . . . . . . . . . <a href="#page-7">7</a>
<a href="#section-2.2">2.2</a>. Header Fields . . . . . . . . . . . . . . . . . . . . . . <a href="#page-8">8</a>
<a href="#section-2.2.1">2.2.1</a>. Unstructured Header Field Bodies . . . . . . . . . . . <a href="#page-8">8</a>
<a href="#section-2.2.2">2.2.2</a>. Structured Header Field Bodies . . . . . . . . . . . . <a href="#page-8">8</a>
<a href="#section-2.2.3">2.2.3</a>. Long Header Fields . . . . . . . . . . . . . . . . . . <a href="#page-8">8</a>
<a href="#section-2.3">2.3</a>. Body . . . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-9">9</a>
<a href="#section-3">3</a>. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-10">10</a>
<a href="#section-3.1">3.1</a>. Introduction . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-10">10</a>
<a href="#section-3.2">3.2</a>. Lexical Tokens . . . . . . . . . . . . . . . . . . . . . . <a href="#page-10">10</a>
<a href="#section-3.2.1">3.2.1</a>. Quoted characters . . . . . . . . . . . . . . . . . . <a href="#page-10">10</a>
<a href="#section-3.2.2">3.2.2</a>. Folding White Space and Comments . . . . . . . . . . . <a href="#page-11">11</a>
<a href="#section-3.2.3">3.2.3</a>. Atom . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-12">12</a>
<a href="#section-3.2.4">3.2.4</a>. Quoted Strings . . . . . . . . . . . . . . . . . . . . <a href="#page-13">13</a>
<a href="#section-3.2.5">3.2.5</a>. Miscellaneous Tokens . . . . . . . . . . . . . . . . . <a href="#page-14">14</a>
<a href="#section-3.3">3.3</a>. Date and Time Specification . . . . . . . . . . . . . . . <a href="#page-14">14</a>
<a href="#section-3.4">3.4</a>. Address Specification . . . . . . . . . . . . . . . . . . <a href="#page-16">16</a>
<a href="#section-3.4.1">3.4.1</a>. Addr-Spec Specification . . . . . . . . . . . . . . . <a href="#page-17">17</a>
<a href="#section-3.5">3.5</a>. Overall Message Syntax . . . . . . . . . . . . . . . . . . <a href="#page-18">18</a>
<a href="#section-3.6">3.6</a>. Field Definitions . . . . . . . . . . . . . . . . . . . . <a href="#page-19">19</a>
<a href="#section-3.6.1">3.6.1</a>. The Origination Date Field . . . . . . . . . . . . . . <a href="#page-22">22</a>
<a href="#section-3.6.2">3.6.2</a>. Originator Fields . . . . . . . . . . . . . . . . . . <a href="#page-22">22</a>
<a href="#section-3.6.3">3.6.3</a>. Destination Address Fields . . . . . . . . . . . . . . <a href="#page-23">23</a>
<a href="#section-3.6.4">3.6.4</a>. Identification Fields . . . . . . . . . . . . . . . . <a href="#page-25">25</a>
<a href="#section-3.6.5">3.6.5</a>. Informational Fields . . . . . . . . . . . . . . . . . <a href="#page-27">27</a>
<a href="#section-3.6.6">3.6.6</a>. Resent Fields . . . . . . . . . . . . . . . . . . . . <a href="#page-28">28</a>
<a href="#section-3.6.7">3.6.7</a>. Trace Fields . . . . . . . . . . . . . . . . . . . . . <a href="#page-30">30</a>
<a href="#section-3.6.8">3.6.8</a>. Optional Fields . . . . . . . . . . . . . . . . . . . <a href="#page-30">30</a>
<a href="#section-4">4</a>. Obsolete Syntax . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-31">31</a>
<a href="#section-4.1">4.1</a>. Miscellaneous Obsolete Tokens . . . . . . . . . . . . . . <a href="#page-32">32</a>
<a href="#section-4.2">4.2</a>. Obsolete Folding White Space . . . . . . . . . . . . . . . <a href="#page-33">33</a>
<a href="#section-4.3">4.3</a>. Obsolete Date and Time . . . . . . . . . . . . . . . . . . <a href="#page-33">33</a>
<a href="#section-4.4">4.4</a>. Obsolete Addressing . . . . . . . . . . . . . . . . . . . <a href="#page-35">35</a>
<a href="#section-4.5">4.5</a>. Obsolete Header Fields . . . . . . . . . . . . . . . . . . <a href="#page-35">35</a>
<a href="#section-4.5.1">4.5.1</a>. Obsolete Origination Date Field . . . . . . . . . . . <a href="#page-36">36</a>
<a href="#section-4.5.2">4.5.2</a>. Obsolete Originator Fields . . . . . . . . . . . . . . <a href="#page-36">36</a>
<a href="#section-4.5.3">4.5.3</a>. Obsolete Destination Address Fields . . . . . . . . . <a href="#page-37">37</a>
<a href="#section-4.5.4">4.5.4</a>. Obsolete Identification Fields . . . . . . . . . . . . <a href="#page-37">37</a>
<a href="#section-4.5.5">4.5.5</a>. Obsolete Informational Fields . . . . . . . . . . . . <a href="#page-37">37</a>
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<a href="#section-4.5.6">4.5.6</a>. Obsolete Resent Fields . . . . . . . . . . . . . . . . <a href="#page-38">38</a>
<a href="#section-4.5.7">4.5.7</a>. Obsolete Trace Fields . . . . . . . . . . . . . . . . <a href="#page-38">38</a>
<a href="#section-4.5.8">4.5.8</a>. Obsolete optional fields . . . . . . . . . . . . . . . <a href="#page-38">38</a>
<a href="#section-5">5</a>. Security Considerations . . . . . . . . . . . . . . . . . . . <a href="#page-38">38</a>
<a href="#section-6">6</a>. IANA Considerations . . . . . . . . . . . . . . . . . . . . . <a href="#page-39">39</a>
<a href="#appendix-A">Appendix A</a>. Example Messages . . . . . . . . . . . . . . . . . <a href="#page-43">43</a>
<a href="#appendix-A.1">Appendix A.1</a>. Addressing Examples . . . . . . . . . . . . . . . <a href="#page-44">44</a>
<a href="#appendix-A.1.1">Appendix A.1.1</a>. A Message from One Person to Another with
Simple Addressing . . . . . . . . . . . . . . . . <a href="#page-44">44</a>
<a href="#appendix-A.1.2">Appendix A.1.2</a>. Different Types of Mailboxes . . . . . . . . . . . <a href="#page-45">45</a>
<a href="#appendix-A.1.3">Appendix A.1.3</a>. Group Addresses . . . . . . . . . . . . . . . . . <a href="#page-45">45</a>
<a href="#appendix-A.2">Appendix A.2</a>. Reply Messages . . . . . . . . . . . . . . . . . . <a href="#page-46">46</a>
<a href="#appendix-A.3">Appendix A.3</a>. Resent Messages . . . . . . . . . . . . . . . . . <a href="#page-47">47</a>
<a href="#appendix-A.4">Appendix A.4</a>. Messages with Trace Fields . . . . . . . . . . . . <a href="#page-48">48</a>
<a href="#appendix-A.5">Appendix A.5</a>. White Space, Comments, and Other Oddities . . . . <a href="#page-49">49</a>
<a href="#appendix-A.6">Appendix A.6</a>. Obsoleted Forms . . . . . . . . . . . . . . . . . <a href="#page-50">50</a>
<a href="#appendix-A.6.1">Appendix A.6.1</a>. Obsolete Addressing . . . . . . . . . . . . . . . <a href="#page-50">50</a>
<a href="#appendix-A.6.2">Appendix A.6.2</a>. Obsolete Dates . . . . . . . . . . . . . . . . . . <a href="#page-50">50</a>
<a href="#appendix-A.6.3">Appendix A.6.3</a>. Obsolete White Space and Comments . . . . . . . . <a href="#page-51">51</a>
<a href="#appendix-B">Appendix B</a>. Differences from Earlier Specifications . . . . . <a href="#page-52">52</a>
<a href="#appendix-C">Appendix C</a>. Acknowledgements . . . . . . . . . . . . . . . . . <a href="#page-53">53</a>
<a href="#section-7">7</a>. References . . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-55">55</a>
<a href="#section-7.1">7.1</a>. Normative References . . . . . . . . . . . . . . . . . . . <a href="#page-55">55</a>
<a href="#section-7.2">7.2</a>. Informative References . . . . . . . . . . . . . . . . . . <a href="#page-55">55</a>
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<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>. Introduction</span>
<span class="h3"><a class="selflink" id="section-1.1" href="#section-1.1">1.1</a>. Scope</span>
This document specifies the Internet Message Format (IMF), a syntax
for text messages that are sent between computer users, within the
framework of "electronic mail" messages. This specification is an
update to [<a href="./rfc2822" title=""Internet Message Format"">RFC2822</a>], which itself superseded [<a href="./rfc0822" title=""Standard for the format of ARPA Internet text messages"">RFC0822</a>], updating it
to reflect current practice and incorporating incremental changes
that were specified in other RFCs such as [<a href="./rfc1123" title=""Requirements for Internet Hosts - Application and Support"">RFC1123</a>].
This document specifies a syntax only for text messages. In
particular, it makes no provision for the transmission of images,
audio, or other sorts of structured data in electronic mail messages.
There are several extensions published, such as the MIME document
series ([<a href="./rfc2045" title=""Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies"">RFC2045</a>], [<a href="./rfc2046" title=""Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types"">RFC2046</a>], [<a href="./rfc2049" title=""Multipurpose Internet Mail Extensions (MIME) Part Five: Conformance Criteria and Examples"">RFC2049</a>]), which describe mechanisms
for the transmission of such data through electronic mail, either by
extending the syntax provided here or by structuring such messages to
conform to this syntax. Those mechanisms are outside of the scope of
this specification.
In the context of electronic mail, messages are viewed as having an
envelope and contents. The envelope contains whatever information is
needed to accomplish transmission and delivery. (See [<a href="./rfc5321" title=""Simple Mail Transfer Protocol"">RFC5321</a>] for a
discussion of the envelope.) The contents comprise the object to be
delivered to the recipient. This specification applies only to the
format and some of the semantics of message contents. It contains no
specification of the information in the envelope.
However, some message systems may use information from the contents
to create the envelope. It is intended that this specification
facilitate the acquisition of such information by programs.
This specification is intended as a definition of what message
content format is to be passed between systems. Though some message
systems locally store messages in this format (which eliminates the
need for translation between formats) and others use formats that
differ from the one specified in this specification, local storage is
outside of the scope of this specification.
Note: This specification is not intended to dictate the internal
formats used by sites, the specific message system features that
they are expected to support, or any of the characteristics of
user interface programs that create or read messages. In
addition, this document does not specify an encoding of the
characters for either transport or storage; that is, it does not
specify the number of bits used or how those bits are specifically
transferred over the wire or stored on disk.
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<span class="h3"><a class="selflink" id="section-1.2" href="#section-1.2">1.2</a>. Notational Conventions</span>
<span class="h4"><a class="selflink" id="section-1.2.1" href="#section-1.2.1">1.2.1</a>. Requirements Notation</span>
This document occasionally uses terms that appear in capital letters.
When the terms "MUST", "SHOULD", "RECOMMENDED", "MUST NOT", "SHOULD
NOT", and "MAY" appear capitalized, they are being used to indicate
particular requirements of this specification. A discussion of the
meanings of these terms appears in [<a href="./rfc2119" title=""Key words for use in RFCs to Indicate Requirement Levels"">RFC2119</a>].
<span class="h4"><a class="selflink" id="section-1.2.2" href="#section-1.2.2">1.2.2</a>. Syntactic Notation</span>
This specification uses the Augmented Backus-Naur Form (ABNF)
[<a href="./rfc5234" title=""Augmented BNF for Syntax Specifications: ABNF"">RFC5234</a>] notation for the formal definitions of the syntax of
messages. Characters will be specified either by a decimal value
(e.g., the value %d65 for uppercase A and %d97 for lowercase A) or by
a case-insensitive literal value enclosed in quotation marks (e.g.,
"A" for either uppercase or lowercase A).
<span class="h4"><a class="selflink" id="section-1.2.3" href="#section-1.2.3">1.2.3</a>. Structure of This Document</span>
This document is divided into several sections.
This section, <a href="#section-1">section 1</a>, is a short introduction to the document.
<a href="#section-2">Section 2</a> lays out the general description of a message and its
constituent parts. This is an overview to help the reader understand
some of the general principles used in the later portions of this
document. Any examples in this section MUST NOT be taken as
specification of the formal syntax of any part of a message.
<a href="#section-3">Section 3</a> specifies formal ABNF rules for the structure of each part
of a message (the syntax) and describes the relationship between
those parts and their meaning in the context of a message (the
semantics). That is, it lays out the actual rules for the structure
of each part of a message (the syntax) as well as a description of
the parts and instructions for their interpretation (the semantics).
This includes analysis of the syntax and semantics of subparts of
messages that have specific structure. The syntax included in
<a href="#section-3">section 3</a> represents messages as they MUST be created. There are
also notes in <a href="#section-3">section 3</a> to indicate if any of the options specified
in the syntax SHOULD be used over any of the others.
Both sections <a href="#section-2">2</a> and <a href="#section-3">3</a> describe messages that are legal to generate
for purposes of this specification.
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<a href="#section-4">Section 4</a> of this document specifies an "obsolete" syntax. There are
references in <a href="#section-3">section 3</a> to these obsolete syntactic elements. The
rules of the obsolete syntax are elements that have appeared in
earlier versions of this specification or have previously been widely
used in Internet messages. As such, these elements MUST be
interpreted by parsers of messages in order to be conformant to this
specification. However, since items in this syntax have been
determined to be non-interoperable or to cause significant problems
for recipients of messages, they MUST NOT be generated by creators of
conformant messages.
<a href="#section-5">Section 5</a> details security considerations to take into account when
implementing this specification.
<a href="#appendix-A">Appendix A</a> lists examples of different sorts of messages. These
examples are not exhaustive of the types of messages that appear on
the Internet, but give a broad overview of certain syntactic forms.
<a href="#appendix-B">Appendix B</a> lists the differences between this specification and
earlier specifications for Internet messages.
<a href="#appendix-C">Appendix C</a> contains acknowledgements.
<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>. Lexical Analysis of Messages</span>
<span class="h3"><a class="selflink" id="section-2.1" href="#section-2.1">2.1</a>. General Description</span>
At the most basic level, a message is a series of characters. A
message that is conformant with this specification is composed of
characters with values in the range of 1 through 127 and interpreted
as US-ASCII [<a href="#ref-ANSI.X3-4.1986" title=""Coded Character Set - 7-bit American Standard Code for Information Interchange"">ANSI.X3-4.1986</a>] characters. For brevity, this document
sometimes refers to this range of characters as simply "US-ASCII
characters".
Note: This document specifies that messages are made up of
characters in the US-ASCII range of 1 through 127. There are
other documents, specifically the MIME document series ([<a href="./rfc2045" title=""Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies"">RFC2045</a>],
[<a href="./rfc2046" title=""Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types"">RFC2046</a>], [<a href="./rfc2047" title=""MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text"">RFC2047</a>], [<a href="./rfc2049" title=""Multipurpose Internet Mail Extensions (MIME) Part Five: Conformance Criteria and Examples"">RFC2049</a>], [<a href="./rfc4288" title=""Media Type Specifications and Registration Procedures"">RFC4288</a>], [<a href="./rfc4289" title=""Multipurpose Internet Mail Extensions (MIME) Part Four: Registration Procedures"">RFC4289</a>]), that
extend this specification to allow for values outside of that
range. Discussion of those mechanisms is not within the scope of
this specification.
Messages are divided into lines of characters. A line is a series of
characters that is delimited with the two characters carriage-return
and line-feed; that is, the carriage return (CR) character (ASCII
value 13) followed immediately by the line feed (LF) character (ASCII
value 10). (The carriage return/line feed pair is usually written in
this document as "CRLF".)
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A message consists of header fields (collectively called "the header
section of the message") followed, optionally, by a body. The header
section is a sequence of lines of characters with special syntax as
defined in this specification. The body is simply a sequence of
characters that follows the header section and is separated from the
header section by an empty line (i.e., a line with nothing preceding
the CRLF).
Note: Common parlance and earlier versions of this specification
use the term "header" to either refer to the entire header section
or to refer to an individual header field. To avoid ambiguity,
this document does not use the terms "header" or "headers" in
isolation, but instead always uses "header field" to refer to the
individual field and "header section" to refer to the entire
collection.
<span class="h4"><a class="selflink" id="section-2.1.1" href="#section-2.1.1">2.1.1</a>. Line Length Limits</span>
There are two limits that this specification places on the number of
characters in a line. Each line of characters MUST be no more than
998 characters, and SHOULD be no more than 78 characters, excluding
the CRLF.
The 998 character limit is due to limitations in many implementations
that send, receive, or store IMF messages which simply cannot handle
more than 998 characters on a line. Receiving implementations would
do well to handle an arbitrarily large number of characters in a line
for robustness sake. However, there are so many implementations that
(in compliance with the transport requirements of [<a href="./rfc5321" title=""Simple Mail Transfer Protocol"">RFC5321</a>]) do not
accept messages containing more than 1000 characters including the CR
and LF per line, it is important for implementations not to create
such messages.
The more conservative 78 character recommendation is to accommodate
the many implementations of user interfaces that display these
messages which may truncate, or disastrously wrap, the display of
more than 78 characters per line, in spite of the fact that such
implementations are non-conformant to the intent of this
specification (and that of [<a href="./rfc5321" title=""Simple Mail Transfer Protocol"">RFC5321</a>] if they actually cause
information to be lost). Again, even though this limitation is put
on messages, it is incumbent upon implementations that display
messages to handle an arbitrarily large number of characters in a
line (certainly at least up to the 998 character limit) for the sake
of robustness.
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<span class="h3"><a class="selflink" id="section-2.2" href="#section-2.2">2.2</a>. Header Fields</span>
Header fields are lines beginning with a field name, followed by a
colon (":"), followed by a field body, and terminated by CRLF. A
field name MUST be composed of printable US-ASCII characters (i.e.,
characters that have values between 33 and 126, inclusive), except
colon. A field body may be composed of printable US-ASCII characters
as well as the space (SP, ASCII value 32) and horizontal tab (HTAB,
ASCII value 9) characters (together known as the white space
characters, WSP). A field body MUST NOT include CR and LF except
when used in "folding" and "unfolding", as described in <a href="#section-2.2.3">section</a>
<a href="#section-2.2.3">2.2.3</a>. All field bodies MUST conform to the syntax described in
sections <a href="#section-3">3</a> and <a href="#section-4">4</a> of this specification.
<span class="h4"><a class="selflink" id="section-2.2.1" href="#section-2.2.1">2.2.1</a>. Unstructured Header Field Bodies</span>
Some field bodies in this specification are defined simply as
"unstructured" (which is specified in <a href="#section-3.2.5">section 3.2.5</a> as any printable
US-ASCII characters plus white space characters) with no further
restrictions. These are referred to as unstructured field bodies.
Semantically, unstructured field bodies are simply to be treated as a
single line of characters with no further processing (except for
"folding" and "unfolding" as described in <a href="#section-2.2.3">section 2.2.3</a>).
<span class="h4"><a class="selflink" id="section-2.2.2" href="#section-2.2.2">2.2.2</a>. Structured Header Field Bodies</span>
Some field bodies in this specification have a syntax that is more
restrictive than the unstructured field bodies described above.
These are referred to as "structured" field bodies. Structured field
bodies are sequences of specific lexical tokens as described in
sections <a href="#section-3">3</a> and <a href="#section-4">4</a> of this specification. Many of these tokens are
allowed (according to their syntax) to be introduced or end with
comments (as described in <a href="#section-3.2.2">section 3.2.2</a>) as well as the white space
characters, and those white space characters are subject to "folding"
and "unfolding" as described in <a href="#section-2.2.3">section 2.2.3</a>. Semantic analysis of
structured field bodies is given along with their syntax.
<span class="h4"><a class="selflink" id="section-2.2.3" href="#section-2.2.3">2.2.3</a>. Long Header Fields</span>
Each header field is logically a single line of characters comprising
the field name, the colon, and the field body. For convenience
however, and to deal with the 998/78 character limitations per line,
the field body portion of a header field can be split into a
multiple-line representation; this is called "folding". The general
rule is that wherever this specification allows for folding white
space (not simply WSP characters), a CRLF may be inserted before any
WSP.
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For example, the header field:
Subject: This is a test
can be represented as:
Subject: This
is a test
Note: Though structured field bodies are defined in such a way
that folding can take place between many of the lexical tokens
(and even within some of the lexical tokens), folding SHOULD be
limited to placing the CRLF at higher-level syntactic breaks. For
instance, if a field body is defined as comma-separated values, it
is recommended that folding occur after the comma separating the
structured items in preference to other places where the field
could be folded, even if it is allowed elsewhere.
The process of moving from this folded multiple-line representation
of a header field to its single line representation is called
"unfolding". Unfolding is accomplished by simply removing any CRLF
that is immediately followed by WSP. Each header field should be
treated in its unfolded form for further syntactic and semantic
evaluation. An unfolded header field has no length restriction and
therefore may be indeterminately long.
<span class="h3"><a class="selflink" id="section-2.3" href="#section-2.3">2.3</a>. Body</span>
The body of a message is simply lines of US-ASCII characters. The
only two limitations on the body are as follows:
o CR and LF MUST only occur together as CRLF; they MUST NOT appear
independently in the body.
o Lines of characters in the body MUST be limited to 998 characters,
and SHOULD be limited to 78 characters, excluding the CRLF.
Note: As was stated earlier, there are other documents,
specifically the MIME documents ([<a href="./rfc2045" title=""Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies"">RFC2045</a>], [<a href="./rfc2046" title=""Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types"">RFC2046</a>], [<a href="./rfc2049" title=""Multipurpose Internet Mail Extensions (MIME) Part Five: Conformance Criteria and Examples"">RFC2049</a>],
[<a href="./rfc4288" title=""Media Type Specifications and Registration Procedures"">RFC4288</a>], [<a href="./rfc4289" title=""Multipurpose Internet Mail Extensions (MIME) Part Four: Registration Procedures"">RFC4289</a>]), that extend (and limit) this specification
to allow for different sorts of message bodies. Again, these
mechanisms are beyond the scope of this document.
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<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>. Syntax</span>
<span class="h3"><a class="selflink" id="section-3.1" href="#section-3.1">3.1</a>. Introduction</span>
The syntax as given in this section defines the legal syntax of
Internet messages. Messages that are conformant to this
specification MUST conform to the syntax in this section. If there
are options in this section where one option SHOULD be generated,
that is indicated either in the prose or in a comment next to the
syntax.
For the defined expressions, a short description of the syntax and
use is given, followed by the syntax in ABNF, followed by a semantic
analysis. The following primitive tokens that are used but otherwise
unspecified are taken from the "Core Rules" of [<a href="./rfc5234" title=""Augmented BNF for Syntax Specifications: ABNF"">RFC5234</a>], <a href="#appendix-B.1">Appendix</a>
<a href="#appendix-B.1">B.1</a>: CR, LF, CRLF, HTAB, SP, WSP, DQUOTE, DIGIT, ALPHA, and VCHAR.
In some of the definitions, there will be non-terminals whose names
start with "obs-". These "obs-" elements refer to tokens defined in
the obsolete syntax in <a href="#section-4">section 4</a>. In all cases, these productions
are to be ignored for the purposes of generating legal Internet
messages and MUST NOT be used as part of such a message. However,
when interpreting messages, these tokens MUST be honored as part of
the legal syntax. In this sense, <a href="#section-3">section 3</a> defines a grammar for the
generation of messages, with "obs-" elements that are to be ignored,
while <a href="#section-4">section 4</a> adds grammar for the interpretation of messages.
<span class="h3"><a class="selflink" id="section-3.2" href="#section-3.2">3.2</a>. Lexical Tokens</span>
The following rules are used to define an underlying lexical
analyzer, which feeds tokens to the higher-level parsers. This
section defines the tokens used in structured header field bodies.
Note: Readers of this specification need to pay special attention
to how these lexical tokens are used in both the lower-level and
higher-level syntax later in the document. Particularly, the
white space tokens and the comment tokens defined in <a href="#section-3.2.2">section 3.2.2</a>
get used in the lower-level tokens defined here, and those lower-
level tokens are in turn used as parts of the higher-level tokens
defined later. Therefore, white space and comments may be allowed
in the higher-level tokens even though they may not explicitly
appear in a particular definition.
<span class="h4"><a class="selflink" id="section-3.2.1" href="#section-3.2.1">3.2.1</a>. Quoted characters</span>
Some characters are reserved for special interpretation, such as
delimiting lexical tokens. To permit use of these characters as
uninterpreted data, a quoting mechanism is provided.
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quoted-pair = ("\" (VCHAR / WSP)) / obs-qp
Where any quoted-pair appears, it is to be interpreted as the
character alone. That is to say, the "\" character that appears as
part of a quoted-pair is semantically "invisible".
Note: The "\" character may appear in a message where it is not
part of a quoted-pair. A "\" character that does not appear in a
quoted-pair is not semantically invisible. The only places in
this specification where quoted-pair currently appears are
ccontent, qcontent, and in obs-dtext in <a href="#section-4">section 4</a>.
<span class="h4"><a class="selflink" id="section-3.2.2" href="#section-3.2.2">3.2.2</a>. Folding White Space and Comments</span>
White space characters, including white space used in folding
(described in <a href="#section-2.2.3">section 2.2.3</a>), may appear between many elements in
header field bodies. Also, strings of characters that are treated as
comments may be included in structured field bodies as characters
enclosed in parentheses. The following defines the folding white
space (FWS) and comment constructs.
Strings of characters enclosed in parentheses are considered comments
so long as they do not appear within a "quoted-string", as defined in
<a href="#section-3.2.4">section 3.2.4</a>. Comments may nest.
There are several places in this specification where comments and FWS
may be freely inserted. To accommodate that syntax, an additional
token for "CFWS" is defined for places where comments and/or FWS can
occur. However, where CFWS occurs in this specification, it MUST NOT
be inserted in such a way that any line of a folded header field is
made up entirely of WSP characters and nothing else.
FWS = ([*WSP CRLF] 1*WSP) / obs-FWS
; Folding white space
ctext = %d33-39 / ; Printable US-ASCII
%d42-91 / ; characters not including
%d93-126 / ; "(", ")", or "\"
obs-ctext
ccontent = ctext / quoted-pair / comment
comment = "(" *([FWS] ccontent) [FWS] ")"
CFWS = (1*([FWS] comment) [FWS]) / FWS
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Throughout this specification, where FWS (the folding white space
token) appears, it indicates a place where folding, as discussed in
<a href="#section-2.2.3">section 2.2.3</a>, may take place. Wherever folding appears in a message
(that is, a header field body containing a CRLF followed by any WSP),
unfolding (removal of the CRLF) is performed before any further
semantic analysis is performed on that header field according to this
specification. That is to say, any CRLF that appears in FWS is
semantically "invisible".
A comment is normally used in a structured field body to provide some
human-readable informational text. Since a comment is allowed to
contain FWS, folding is permitted within the comment. Also note that
since quoted-pair is allowed in a comment, the parentheses and
backslash characters may appear in a comment, so long as they appear
as a quoted-pair. Semantically, the enclosing parentheses are not
part of the comment; the comment is what is contained between the two
parentheses. As stated earlier, the "\" in any quoted-pair and the
CRLF in any FWS that appears within the comment are semantically
"invisible" and therefore not part of the comment either.
Runs of FWS, comment, or CFWS that occur between lexical tokens in a
structured header field are semantically interpreted as a single
space character.
<span class="h4"><a class="selflink" id="section-3.2.3" href="#section-3.2.3">3.2.3</a>. Atom</span>
Several productions in structured header field bodies are simply
strings of certain basic characters. Such productions are called
atoms.
Some of the structured header field bodies also allow the period
character (".", ASCII value 46) within runs of atext. An additional
"dot-atom" token is defined for those purposes.
Note: The "specials" token does not appear anywhere else in this
specification. It is simply the visible (i.e., non-control, non-
white space) characters that do not appear in atext. It is
provided only because it is useful for implementers who use tools
that lexically analyze messages. Each of the characters in
specials can be used to indicate a tokenization point in lexical
analysis.
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atext = ALPHA / DIGIT / ; Printable US-ASCII
"!" / "#" / ; characters not including
"$" / "%" / ; specials. Used for atoms.
"&" / "'" /
"*" / "+" /
"-" / "/" /
"=" / "?" /
"^" / "_" /
"`" / "{" /
"|" / "}" /
"~"
atom = [CFWS] 1*atext [CFWS]
dot-atom-text = 1*atext *("." 1*atext)
dot-atom = [CFWS] dot-atom-text [CFWS]
specials = "(" / ")" / ; Special characters that do
"<" / ">" / ; not appear in atext
"[" / "]" /
":" / ";" /
"@" / "\" /
"," / "." /
DQUOTE
Both atom and dot-atom are interpreted as a single unit, comprising
the string of characters that make it up. Semantically, the optional
comments and FWS surrounding the rest of the characters are not part
of the atom; the atom is only the run of atext characters in an atom,
or the atext and "." characters in a dot-atom.
<span class="h4"><a class="selflink" id="section-3.2.4" href="#section-3.2.4">3.2.4</a>. Quoted Strings</span>
Strings of characters that include characters other than those
allowed in atoms can be represented in a quoted string format, where
the characters are surrounded by quote (DQUOTE, ASCII value 34)
characters.
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qtext = %d33 / ; Printable US-ASCII
%d35-91 / ; characters not including
%d93-126 / ; "\" or the quote character
obs-qtext
qcontent = qtext / quoted-pair
quoted-string = [CFWS]
DQUOTE *([FWS] qcontent) [FWS] DQUOTE
[CFWS]
A quoted-string is treated as a unit. That is, quoted-string is
identical to atom, semantically. Since a quoted-string is allowed to
contain FWS, folding is permitted. Also note that since quoted-pair
is allowed in a quoted-string, the quote and backslash characters may
appear in a quoted-string so long as they appear as a quoted-pair.
Semantically, neither the optional CFWS outside of the quote
characters nor the quote characters themselves are part of the
quoted-string; the quoted-string is what is contained between the two
quote characters. As stated earlier, the "\" in any quoted-pair and
the CRLF in any FWS/CFWS that appears within the quoted-string are
semantically "invisible" and therefore not part of the quoted-string
either.
<span class="h4"><a class="selflink" id="section-3.2.5" href="#section-3.2.5">3.2.5</a>. Miscellaneous Tokens</span>
Three additional tokens are defined: word and phrase for combinations
of atoms and/or quoted-strings, and unstructured for use in
unstructured header fields and in some places within structured
header fields.
word = atom / quoted-string
phrase = 1*word / obs-phrase
unstructured = (*([FWS] VCHAR) *WSP) / obs-unstruct
<span class="h3"><a class="selflink" id="section-3.3" href="#section-3.3">3.3</a>. Date and Time Specification</span>
Date and time values occur in several header fields. This section
specifies the syntax for a full date and time specification. Though
folding white space is permitted throughout the date-time
specification, it is RECOMMENDED that a single space be used in each
place that FWS appears (whether it is required or optional); some
older implementations will not interpret longer sequences of folding
white space correctly.
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date-time = [ day-of-week "," ] date time [CFWS]
day-of-week = ([FWS] day-name) / obs-day-of-week
day-name = "Mon" / "Tue" / "Wed" / "Thu" /
"Fri" / "Sat" / "Sun"
date = day month year
day = ([FWS] 1*2DIGIT FWS) / obs-day
month = "Jan" / "Feb" / "Mar" / "Apr" /
"May" / "Jun" / "Jul" / "Aug" /
"Sep" / "Oct" / "Nov" / "Dec"
year = (FWS 4*DIGIT FWS) / obs-year
time = time-of-day zone
time-of-day = hour ":" minute [ ":" second ]
hour = 2DIGIT / obs-hour
minute = 2DIGIT / obs-minute
second = 2DIGIT / obs-second
zone = (FWS ( "+" / "-" ) 4DIGIT) / obs-zone
The day is the numeric day of the month. The year is any numeric
year 1900 or later.
The time-of-day specifies the number of hours, minutes, and
optionally seconds since midnight of the date indicated.
The date and time-of-day SHOULD express local time.
The zone specifies the offset from Coordinated Universal Time (UTC,
formerly referred to as "Greenwich Mean Time") that the date and
time-of-day represent. The "+" or "-" indicates whether the time-of-
day is ahead of (i.e., east of) or behind (i.e., west of) Universal
Time. The first two digits indicate the number of hours difference
from Universal Time, and the last two digits indicate the number of
additional minutes difference from Universal Time. (Hence, +hhmm
means +(hh * 60 + mm) minutes, and -hhmm means -(hh * 60 + mm)
minutes). The form "+0000" SHOULD be used to indicate a time zone at
Universal Time. Though "-0000" also indicates Universal Time, it is
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used to indicate that the time was generated on a system that may be
in a local time zone other than Universal Time and that the date-time
contains no information about the local time zone.
A date-time specification MUST be semantically valid. That is, the
day-of-week (if included) MUST be the day implied by the date, the
numeric day-of-month MUST be between 1 and the number of days allowed
for the specified month (in the specified year), the time-of-day MUST
be in the range 00:00:00 through 23:59:60 (the number of seconds
allowing for a leap second; see [<a href="./rfc1305" title=""Network Time Protocol (Version 3) Specification, Implementation"">RFC1305</a>]), and the last two digits
of the zone MUST be within the range 00 through 59.
<span class="h3"><a class="selflink" id="section-3.4" href="#section-3.4">3.4</a>. Address Specification</span>
Addresses occur in several message header fields to indicate senders
and recipients of messages. An address may either be an individual
mailbox, or a group of mailboxes.
address = mailbox / group
mailbox = name-addr / addr-spec
name-addr = [display-name] angle-addr
angle-addr = [CFWS] "<" addr-spec ">" [CFWS] /
obs-angle-addr
group = display-name ":" [group-list] ";" [CFWS]
display-name = phrase
mailbox-list = (mailbox *("," mailbox)) / obs-mbox-list
address-list = (address *("," address)) / obs-addr-list
group-list = mailbox-list / CFWS / obs-group-list
A mailbox receives mail. It is a conceptual entity that does not
necessarily pertain to file storage. For example, some sites may
choose to print mail on a printer and deliver the output to the
addressee's desk.
Normally, a mailbox is composed of two parts: (1) an optional display
name that indicates the name of the recipient (which can be a person
or a system) that could be displayed to the user of a mail
application, and (2) an addr-spec address enclosed in angle brackets
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("<" and ">"). There is an alternate simple form of a mailbox where
the addr-spec address appears alone, without the recipient's name or
the angle brackets. The Internet addr-spec address is described in
<a href="#section-3.4.1">section 3.4.1</a>.
Note: Some legacy implementations used the simple form where the
addr-spec appears without the angle brackets, but included the
name of the recipient in parentheses as a comment following the
addr-spec. Since the meaning of the information in a comment is
unspecified, implementations SHOULD use the full name-addr form of
the mailbox, instead of the legacy form, to specify the display
name associated with a mailbox. Also, because some legacy
implementations interpret the comment, comments generally SHOULD
NOT be used in address fields to avoid confusing such
implementations.
When it is desirable to treat several mailboxes as a single unit
(i.e., in a distribution list), the group construct can be used. The
group construct allows the sender to indicate a named group of
recipients. This is done by giving a display name for the group,
followed by a colon, followed by a comma-separated list of any number
of mailboxes (including zero and one), and ending with a semicolon.
Because the list of mailboxes can be empty, using the group construct
is also a simple way to communicate to recipients that the message
was sent to one or more named sets of recipients, without actually
providing the individual mailbox address for any of those recipients.
<span class="h4"><a class="selflink" id="section-3.4.1" href="#section-3.4.1">3.4.1</a>. Addr-Spec Specification</span>
An addr-spec is a specific Internet identifier that contains a
locally interpreted string followed by the at-sign character ("@",
ASCII value 64) followed by an Internet domain. The locally
interpreted string is either a quoted-string or a dot-atom. If the
string can be represented as a dot-atom (that is, it contains no
characters other than atext characters or "." surrounded by atext
characters), then the dot-atom form SHOULD be used and the quoted-
string form SHOULD NOT be used. Comments and folding white space
SHOULD NOT be used around the "@" in the addr-spec.
Note: A liberal syntax for the domain portion of addr-spec is
given here. However, the domain portion contains addressing
information specified by and used in other protocols (e.g.,
[<a href="./rfc1034" title=""Domain names - concepts and facilities"">RFC1034</a>], [<a href="./rfc1035" title=""Domain names - implementation and specification"">RFC1035</a>], [<a href="./rfc1123" title=""Requirements for Internet Hosts - Application and Support"">RFC1123</a>], [<a href="./rfc5321" title=""Simple Mail Transfer Protocol"">RFC5321</a>]). It is therefore
incumbent upon implementations to conform to the syntax of
addresses for the context in which they are used.
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addr-spec = local-part "@" domain
local-part = dot-atom / quoted-string / obs-local-part
domain = dot-atom / domain-literal / obs-domain
domain-literal = [CFWS] "[" *([FWS] dtext) [FWS] "]" [CFWS]
dtext = %d33-90 / ; Printable US-ASCII
%d94-126 / ; characters not including
obs-dtext ; "[", "]", or "\"
The domain portion identifies the point to which the mail is
delivered. In the dot-atom form, this is interpreted as an Internet
domain name (either a host name or a mail exchanger name) as
described in [<a href="./rfc1034" title=""Domain names - concepts and facilities"">RFC1034</a>], [<a href="./rfc1035" title=""Domain names - implementation and specification"">RFC1035</a>], and [<a href="./rfc1123" title=""Requirements for Internet Hosts - Application and Support"">RFC1123</a>]. In the domain-
literal form, the domain is interpreted as the literal Internet
address of the particular host. In both cases, how addressing is
used and how messages are transported to a particular host is covered
in separate documents, such as [<a href="./rfc5321" title=""Simple Mail Transfer Protocol"">RFC5321</a>]. These mechanisms are
outside of the scope of this document.
The local-part portion is a domain-dependent string. In addresses,
it is simply interpreted on the particular host as a name of a
particular mailbox.
<span class="h3"><a class="selflink" id="section-3.5" href="#section-3.5">3.5</a>. Overall Message Syntax</span>
A message consists of header fields, optionally followed by a message
body. Lines in a message MUST be a maximum of 998 characters
excluding the CRLF, but it is RECOMMENDED that lines be limited to 78
characters excluding the CRLF. (See <a href="#section-2.1.1">section 2.1.1</a> for explanation.)
In a message body, though all of the characters listed in the text
rule MAY be used, the use of US-ASCII control characters (values 1
through 8, 11, 12, and 14 through 31) is discouraged since their
interpretation by receivers for display is not guaranteed.
message = (fields / obs-fields)
[CRLF body]
body = (*(*998text CRLF) *998text) / obs-body
text = %d1-9 / ; Characters excluding CR
%d11 / ; and LF
%d12 /
%d14-127
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The header fields carry most of the semantic information and are
defined in <a href="#section-3.6">section 3.6</a>. The body is simply a series of lines of text
that are uninterpreted for the purposes of this specification.
<span class="h3"><a class="selflink" id="section-3.6" href="#section-3.6">3.6</a>. Field Definitions</span>
The header fields of a message are defined here. All header fields
have the same general syntactic structure: a field name, followed by
a colon, followed by the field body. The specific syntax for each
header field is defined in the subsequent sections.
Note: In the ABNF syntax for each field in subsequent sections,
each field name is followed by the required colon. However, for
brevity, sometimes the colon is not referred to in the textual
description of the syntax. It is, nonetheless, required.
It is important to note that the header fields are not guaranteed to
be in a particular order. They may appear in any order, and they
have been known to be reordered occasionally when transported over
the Internet. However, for the purposes of this specification,
header fields SHOULD NOT be reordered when a message is transported
or transformed. More importantly, the trace header fields and resent
header fields MUST NOT be reordered, and SHOULD be kept in blocks
prepended to the message. See sections <a href="#section-3.6.6">3.6.6</a> and <a href="#section-3.6.7">3.6.7</a> for more
information.
The only required header fields are the origination date field and
the originator address field(s). All other header fields are
syntactically optional. More information is contained in the table
following this definition.
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fields = *(trace
*optional-field /
*(resent-date /
resent-from /
resent-sender /
resent-to /
resent-cc /
resent-bcc /
resent-msg-id))
*(orig-date /
from /
sender /
reply-to /
to /
cc /
bcc /
message-id /
in-reply-to /
references /
subject /
comments /
keywords /
optional-field)
The following table indicates limits on the number of times each
field may occur in the header section of a message as well as any
special limitations on the use of those fields. An asterisk ("*")
next to a value in the minimum or maximum column indicates that a
special restriction appears in the Notes column.
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+----------------+--------+------------+----------------------------+
| Field | Min | Max number | Notes |
| | number | | |
+----------------+--------+------------+----------------------------+
| trace | 0 | unlimited | Block prepended - see |
| | | | 3.6.7 |
| resent-date | 0* | unlimited* | One per block, required if |
| | | | other resent fields are |
| | | | present - see 3.6.6 |
| resent-from | 0 | unlimited* | One per block - see 3.6.6 |
| resent-sender | 0* | unlimited* | One per block, MUST occur |
| | | | with multi-address |
| | | | resent-from - see 3.6.6 |
| resent-to | 0 | unlimited* | One per block - see 3.6.6 |
| resent-cc | 0 | unlimited* | One per block - see 3.6.6 |
| resent-bcc | 0 | unlimited* | One per block - see 3.6.6 |
| resent-msg-id | 0 | unlimited* | One per block - see 3.6.6 |
| orig-date | 1 | 1 | |
| from | 1 | 1 | See sender and 3.6.2 |
| sender | 0* | 1 | MUST occur with |
| | | | multi-address from - see |
| | | | 3.6.2 |
| reply-to | 0 | 1 | |
| to | 0 | 1 | |
| cc | 0 | 1 | |
| bcc | 0 | 1 | |
| message-id | 0* | 1 | SHOULD be present - see |
| | | | 3.6.4 |
| in-reply-to | 0* | 1 | SHOULD occur in some |
| | | | replies - see 3.6.4 |
| references | 0* | 1 | SHOULD occur in some |
| | | | replies - see 3.6.4 |
| subject | 0 | 1 | |
| comments | 0 | unlimited | |
| keywords | 0 | unlimited | |
| optional-field | 0 | unlimited | |
+----------------+--------+------------+----------------------------+
The exact interpretation of each field is described in subsequent
sections.
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<span class="h4"><a class="selflink" id="section-3.6.1" href="#section-3.6.1">3.6.1</a>. The Origination Date Field</span>
The origination date field consists of the field name "Date" followed
by a date-time specification.
orig-date = "Date:" date-time CRLF
The origination date specifies the date and time at which the creator
of the message indicated that the message was complete and ready to
enter the mail delivery system. For instance, this might be the time
that a user pushes the "send" or "submit" button in an application
program. In any case, it is specifically not intended to convey the
time that the message is actually transported, but rather the time at
which the human or other creator of the message has put the message
into its final form, ready for transport. (For example, a portable
computer user who is not connected to a network might queue a message
for delivery. The origination date is intended to contain the date
and time that the user queued the message, not the time when the user
connected to the network to send the message.)
<span class="h4"><a class="selflink" id="section-3.6.2" href="#section-3.6.2">3.6.2</a>. Originator Fields</span>
The originator fields of a message consist of the from field, the
sender field (when applicable), and optionally the reply-to field.
The from field consists of the field name "From" and a comma-
separated list of one or more mailbox specifications. If the from
field contains more than one mailbox specification in the mailbox-
list, then the sender field, containing the field name "Sender" and a
single mailbox specification, MUST appear in the message. In either
case, an optional reply-to field MAY also be included, which contains
the field name "Reply-To" and a comma-separated list of one or more
addresses.
from = "From:" mailbox-list CRLF
sender = "Sender:" mailbox CRLF
reply-to = "Reply-To:" address-list CRLF
The originator fields indicate the mailbox(es) of the source of the
message. The "From:" field specifies the author(s) of the message,
that is, the mailbox(es) of the person(s) or system(s) responsible
for the writing of the message. The "Sender:" field specifies the
mailbox of the agent responsible for the actual transmission of the
message. For example, if a secretary were to send a message for
another person, the mailbox of the secretary would appear in the
"Sender:" field and the mailbox of the actual author would appear in
the "From:" field. If the originator of the message can be indicated
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by a single mailbox and the author and transmitter are identical, the
"Sender:" field SHOULD NOT be used. Otherwise, both fields SHOULD
appear.
Note: The transmitter information is always present. The absence
of the "Sender:" field is sometimes mistakenly taken to mean that
the agent responsible for transmission of the message has not been
specified. This absence merely means that the transmitter is
identical to the author and is therefore not redundantly placed
into the "Sender:" field.
The originator fields also provide the information required when
replying to a message. When the "Reply-To:" field is present, it
indicates the address(es) to which the author of the message suggests
that replies be sent. In the absence of the "Reply-To:" field,
replies SHOULD by default be sent to the mailbox(es) specified in the
"From:" field unless otherwise specified by the person composing the
reply.
In all cases, the "From:" field SHOULD NOT contain any mailbox that
does not belong to the author(s) of the message. See also <a href="#section-3.6.3">section</a>
<a href="#section-3.6.3">3.6.3</a> for more information on forming the destination addresses for a
reply.
<span class="h4"><a class="selflink" id="section-3.6.3" href="#section-3.6.3">3.6.3</a>. Destination Address Fields</span>
The destination fields of a message consist of three possible fields,
each of the same form: the field name, which is either "To", "Cc", or
"Bcc", followed by a comma-separated list of one or more addresses
(either mailbox or group syntax).
to = "To:" address-list CRLF
cc = "Cc:" address-list CRLF
bcc = "Bcc:" [address-list / CFWS] CRLF
The destination fields specify the recipients of the message. Each
destination field may have one or more addresses, and the addresses
indicate the intended recipients of the message. The only difference
between the three fields is how each is used.
The "To:" field contains the address(es) of the primary recipient(s)
of the message.
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The "Cc:" field (where the "Cc" means "Carbon Copy" in the sense of
making a copy on a typewriter using carbon paper) contains the
addresses of others who are to receive the message, though the
content of the message may not be directed at them.
The "Bcc:" field (where the "Bcc" means "Blind Carbon Copy") contains
addresses of recipients of the message whose addresses are not to be
revealed to other recipients of the message. There are three ways in
which the "Bcc:" field is used. In the first case, when a message
containing a "Bcc:" field is prepared to be sent, the "Bcc:" line is
removed even though all of the recipients (including those specified
in the "Bcc:" field) are sent a copy of the message. In the second
case, recipients specified in the "To:" and "Cc:" lines each are sent
a copy of the message with the "Bcc:" line removed as above, but the
recipients on the "Bcc:" line get a separate copy of the message
containing a "Bcc:" line. (When there are multiple recipient
addresses in the "Bcc:" field, some implementations actually send a
separate copy of the message to each recipient with a "Bcc:"
containing only the address of that particular recipient.) Finally,
since a "Bcc:" field may contain no addresses, a "Bcc:" field can be
sent without any addresses indicating to the recipients that blind
copies were sent to someone. Which method to use with "Bcc:" fields
is implementation dependent, but refer to the "Security
Considerations" section of this document for a discussion of each.
When a message is a reply to another message, the mailboxes of the
authors of the original message (the mailboxes in the "From:" field)
or mailboxes specified in the "Reply-To:" field (if it exists) MAY
appear in the "To:" field of the reply since these would normally be
the primary recipients of the reply. If a reply is sent to a message
that has destination fields, it is often desirable to send a copy of
the reply to all of the recipients of the message, in addition to the
author. When such a reply is formed, addresses in the "To:" and
"Cc:" fields of the original message MAY appear in the "Cc:" field of
the reply, since these are normally secondary recipients of the
reply. If a "Bcc:" field is present in the original message,
addresses in that field MAY appear in the "Bcc:" field of the reply,
but they SHOULD NOT appear in the "To:" or "Cc:" fields.
Note: Some mail applications have automatic reply commands that
include the destination addresses of the original message in the
destination addresses of the reply. How those reply commands
behave is implementation dependent and is beyond the scope of this
document. In particular, whether or not to include the original
destination addresses when the original message had a "Reply-To:"
field is not addressed here.
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<span class="h4"><a class="selflink" id="section-3.6.4" href="#section-3.6.4">3.6.4</a>. Identification Fields</span>
Though listed as optional in the table in <a href="#section-3.6">section 3.6</a>, every message
SHOULD have a "Message-ID:" field. Furthermore, reply messages
SHOULD have "In-Reply-To:" and "References:" fields as appropriate
and as described below.
The "Message-ID:" field contains a single unique message identifier.
The "References:" and "In-Reply-To:" fields each contain one or more
unique message identifiers, optionally separated by CFWS.
The message identifier (msg-id) syntax is a limited version of the
addr-spec construct enclosed in the angle bracket characters, "<" and
">". Unlike addr-spec, this syntax only permits the dot-atom-text
form on the left-hand side of the "@" and does not have internal CFWS
anywhere in the message identifier.
Note: As with addr-spec, a liberal syntax is given for the right-
hand side of the "@" in a msg-id. However, later in this section,
the use of a domain for the right-hand side of the "@" is
RECOMMENDED. Again, the syntax of domain constructs is specified
by and used in other protocols (e.g., [<a href="./rfc1034" title=""Domain names - concepts and facilities"">RFC1034</a>], [<a href="./rfc1035" title=""Domain names - implementation and specification"">RFC1035</a>],
[<a href="./rfc1123" title=""Requirements for Internet Hosts - Application and Support"">RFC1123</a>], [<a href="./rfc5321" title=""Simple Mail Transfer Protocol"">RFC5321</a>]). It is therefore incumbent upon
implementations to conform to the syntax of addresses for the
context in which they are used.
message-id = "Message-ID:" msg-id CRLF
in-reply-to = "In-Reply-To:" 1*msg-id CRLF
references = "References:" 1*msg-id CRLF
msg-id = [CFWS] "<" id-left "@" id-right ">" [CFWS]
id-left = dot-atom-text / obs-id-left
id-right = dot-atom-text / no-fold-literal / obs-id-right
no-fold-literal = "[" *dtext "]"
The "Message-ID:" field provides a unique message identifier that
refers to a particular version of a particular message. The
uniqueness of the message identifier is guaranteed by the host that
generates it (see below). This message identifier is intended to be
machine readable and not necessarily meaningful to humans. A message
identifier pertains to exactly one version of a particular message;
subsequent revisions to the message each receive new message
identifiers.
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Note: There are many instances when messages are "changed", but
those changes do not constitute a new instantiation of that
message, and therefore the message would not get a new message
identifier. For example, when messages are introduced into the
transport system, they are often prepended with additional header
fields such as trace fields (described in <a href="#section-3.6.7">section 3.6.7</a>) and
resent fields (described in <a href="#section-3.6.6">section 3.6.6</a>). The addition of such
header fields does not change the identity of the message and
therefore the original "Message-ID:" field is retained. In all
cases, it is the meaning that the sender of the message wishes to
convey (i.e., whether this is the same message or a different
message) that determines whether or not the "Message-ID:" field
changes, not any particular syntactic difference that appears (or
does not appear) in the message.
The "In-Reply-To:" and "References:" fields are used when creating a
reply to a message. They hold the message identifier of the original
message and the message identifiers of other messages (for example,
in the case of a reply to a message that was itself a reply). The
"In-Reply-To:" field may be used to identify the message (or
messages) to which the new message is a reply, while the
"References:" field may be used to identify a "thread" of
conversation.
When creating a reply to a message, the "In-Reply-To:" and
"References:" fields of the resultant message are constructed as
follows:
The "In-Reply-To:" field will contain the contents of the
"Message-ID:" field of the message to which this one is a reply (the
"parent message"). If there is more than one parent message, then
the "In-Reply-To:" field will contain the contents of all of the
parents' "Message-ID:" fields. If there is no "Message-ID:" field in
any of the parent messages, then the new message will have no "In-
Reply-To:" field.
The "References:" field will contain the contents of the parent's
"References:" field (if any) followed by the contents of the parent's
"Message-ID:" field (if any). If the parent message does not contain
a "References:" field but does have an "In-Reply-To:" field
containing a single message identifier, then the "References:" field
will contain the contents of the parent's "In-Reply-To:" field
followed by the contents of the parent's "Message-ID:" field (if
any). If the parent has none of the "References:", "In-Reply-To:",
or "Message-ID:" fields, then the new message will have no
"References:" field.
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Note: Some implementations parse the "References:" field to
display the "thread of the discussion". These implementations
assume that each new message is a reply to a single parent and
hence that they can walk backwards through the "References:" field
to find the parent of each message listed there. Therefore,
trying to form a "References:" field for a reply that has multiple
parents is discouraged; how to do so is not defined in this
document.
The message identifier (msg-id) itself MUST be a globally unique
identifier for a message. The generator of the message identifier
MUST guarantee that the msg-id is unique. There are several
algorithms that can be used to accomplish this. Since the msg-id has
a similar syntax to addr-spec (identical except that quoted strings,
comments, and folding white space are not allowed), a good method is
to put the domain name (or a domain literal IP address) of the host
on which the message identifier was created on the right-hand side of
the "@" (since domain names and IP addresses are normally unique),
and put a combination of the current absolute date and time along
with some other currently unique (perhaps sequential) identifier
available on the system (for example, a process id number) on the
left-hand side. Though other algorithms will work, it is RECOMMENDED
that the right-hand side contain some domain identifier (either of
the host itself or otherwise) such that the generator of the message
identifier can guarantee the uniqueness of the left-hand side within
the scope of that domain.
Semantically, the angle bracket characters are not part of the
msg-id; the msg-id is what is contained between the two angle bracket
characters.
<span class="h4"><a class="selflink" id="section-3.6.5" href="#section-3.6.5">3.6.5</a>. Informational Fields</span>
The informational fields are all optional. The "Subject:" and
"Comments:" fields are unstructured fields as defined in <a href="#section-2.2.1">section</a>
<a href="#section-2.2.1">2.2.1</a>, and therefore may contain text or folding white space. The
"Keywords:" field contains a comma-separated list of one or more
words or quoted-strings.
subject = "Subject:" unstructured CRLF
comments = "Comments:" unstructured CRLF
keywords = "Keywords:" phrase *("," phrase) CRLF
These three fields are intended to have only human-readable content
with information about the message. The "Subject:" field is the most
common and contains a short string identifying the topic of the
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message. When used in a reply, the field body MAY start with the
string "Re: " (an abbreviation of the Latin "in re", meaning "in the
matter of") followed by the contents of the "Subject:" field body of
the original message. If this is done, only one instance of the
literal string "Re: " ought to be used since use of other strings or
more than one instance can lead to undesirable consequences. The
"Comments:" field contains any additional comments on the text of the
body of the message. The "Keywords:" field contains a comma-
separated list of important words and phrases that might be useful
for the recipient.
<span class="h4"><a class="selflink" id="section-3.6.6" href="#section-3.6.6">3.6.6</a>. Resent Fields</span>
Resent fields SHOULD be added to any message that is reintroduced by
a user into the transport system. A separate set of resent fields
SHOULD be added each time this is done. All of the resent fields
corresponding to a particular resending of the message SHOULD be
grouped together. Each new set of resent fields is prepended to the
message; that is, the most recent set of resent fields appears
earlier in the message. No other fields in the message are changed
when resent fields are added.
Each of the resent fields corresponds to a particular field elsewhere
in the syntax. For instance, the "Resent-Date:" field corresponds to
the "Date:" field and the "Resent-To:" field corresponds to the "To:"
field. In each case, the syntax for the field body is identical to
the syntax given previously for the corresponding field.
When resent fields are used, the "Resent-From:" and "Resent-Date:"
fields MUST be sent. The "Resent-Message-ID:" field SHOULD be sent.
"Resent-Sender:" SHOULD NOT be used if "Resent-Sender:" would be
identical to "Resent-From:".
resent-date = "Resent-Date:" date-time CRLF
resent-from = "Resent-From:" mailbox-list CRLF
resent-sender = "Resent-Sender:" mailbox CRLF
resent-to = "Resent-To:" address-list CRLF
resent-cc = "Resent-Cc:" address-list CRLF
resent-bcc = "Resent-Bcc:" [address-list / CFWS] CRLF
resent-msg-id = "Resent-Message-ID:" msg-id CRLF
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Resent fields are used to identify a message as having been
reintroduced into the transport system by a user. The purpose of
using resent fields is to have the message appear to the final
recipient as if it were sent directly by the original sender, with
all of the original fields remaining the same. Each set of resent
fields correspond to a particular resending event. That is, if a
message is resent multiple times, each set of resent fields gives
identifying information for each individual time. Resent fields are
strictly informational. They MUST NOT be used in the normal
processing of replies or other such automatic actions on messages.
Note: Reintroducing a message into the transport system and using
resent fields is a different operation from "forwarding".
"Forwarding" has two meanings: One sense of forwarding is that a
mail reading program can be told by a user to forward a copy of a
message to another person, making the forwarded message the body
of the new message. A forwarded message in this sense does not
appear to have come from the original sender, but is an entirely
new message from the forwarder of the message. Forwarding may
also mean that a mail transport program gets a message and
forwards it on to a different destination for final delivery.
Resent header fields are not intended for use with either type of
forwarding.
The resent originator fields indicate the mailbox of the person(s) or
system(s) that resent the message. As with the regular originator
fields, there are two forms: a simple "Resent-From:" form, which
contains the mailbox of the individual doing the resending, and the
more complex form, when one individual (identified in the "Resent-
Sender:" field) resends a message on behalf of one or more others
(identified in the "Resent-From:" field).
Note: When replying to a resent message, replies behave just as
they would with any other message, using the original "From:",
"Reply-To:", "Message-ID:", and other fields. The resent fields
are only informational and MUST NOT be used in the normal
processing of replies.
The "Resent-Date:" indicates the date and time at which the resent
message is dispatched by the resender of the message. Like the
"Date:" field, it is not the date and time that the message was
actually transported.
The "Resent-To:", "Resent-Cc:", and "Resent-Bcc:" fields function
identically to the "To:", "Cc:", and "Bcc:" fields, respectively,
except that they indicate the recipients of the resent message, not
the recipients of the original message.
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The "Resent-Message-ID:" field provides a unique identifier for the
resent message.
<span class="h4"><a class="selflink" id="section-3.6.7" href="#section-3.6.7">3.6.7</a>. Trace Fields</span>
The trace fields are a group of header fields consisting of an
optional "Return-Path:" field, and one or more "Received:" fields.
The "Return-Path:" header field contains a pair of angle brackets
that enclose an optional addr-spec. The "Received:" field contains a
(possibly empty) list of tokens followed by a semicolon and a date-
time specification. Each token must be a word, angle-addr, addr-
spec, or a domain. Further restrictions are applied to the syntax of
the trace fields by specifications that provide for their use, such
as [<a href="./rfc5321" title=""Simple Mail Transfer Protocol"">RFC5321</a>].
trace = [return]
1*received
return = "Return-Path:" path CRLF
path = angle-addr / ([CFWS] "<" [CFWS] ">" [CFWS])
received = "Received:" *received-token ";" date-time CRLF
received-token = word / angle-addr / addr-spec / domain
A full discussion of the Internet mail use of trace fields is
contained in [<a href="./rfc5321" title=""Simple Mail Transfer Protocol"">RFC5321</a>]. For the purposes of this specification, the
trace fields are strictly informational, and any formal
interpretation of them is outside of the scope of this document.
<span class="h4"><a class="selflink" id="section-3.6.8" href="#section-3.6.8">3.6.8</a>. Optional Fields</span>
Fields may appear in messages that are otherwise unspecified in this
document. They MUST conform to the syntax of an optional-field.
This is a field name, made up of the printable US-ASCII characters
except SP and colon, followed by a colon, followed by any text that
conforms to the unstructured syntax.
The field names of any optional field MUST NOT be identical to any
field name specified elsewhere in this document.
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optional-field = field-name ":" unstructured CRLF
field-name = 1*ftext
ftext = %d33-57 / ; Printable US-ASCII
%d59-126 ; characters not including
; ":".
For the purposes of this specification, any optional field is
uninterpreted.
<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>. Obsolete Syntax</span>
Earlier versions of this specification allowed for different (usually
more liberal) syntax than is allowed in this version. Also, there
have been syntactic elements used in messages on the Internet whose
interpretations have never been documented. Though these syntactic
forms MUST NOT be generated according to the grammar in <a href="#section-3">section 3</a>,
they MUST be accepted and parsed by a conformant receiver. This
section documents many of these syntactic elements. Taking the
grammar in <a href="#section-3">section 3</a> and adding the definitions presented in this
section will result in the grammar to use for the interpretation of
messages.
Note: This section identifies syntactic forms that any
implementation MUST reasonably interpret. However, there are
certainly Internet messages that do not conform to even the
additional syntax given in this section. The fact that a
particular form does not appear in any section of this document is
not justification for computer programs to crash or for malformed
data to be irretrievably lost by any implementation. It is up to
the implementation to deal with messages robustly.
One important difference between the obsolete (interpreting) and the
current (generating) syntax is that in structured header field bodies
(i.e., between the colon and the CRLF of any structured header
field), white space characters, including folding white space, and
comments could be freely inserted between any syntactic tokens. This
allowed many complex forms that have proven difficult for some
implementations to parse.
Another key difference between the obsolete and the current syntax is
that the rule in <a href="#section-3.2.2">section 3.2.2</a> regarding lines composed entirely of
white space in comments and folding white space does not apply. See
the discussion of folding white space in <a href="#section-4.2">section 4.2</a> below.
Finally, certain characters that were formerly allowed in messages
appear in this section. The NUL character (ASCII value 0) was once
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allowed, but is no longer for compatibility reasons. Similarly, US-
ASCII control characters other than CR, LF, SP, and HTAB (ASCII
values 1 through 8, 11, 12, 14 through 31, and 127) were allowed to
appear in header field bodies. CR and LF were allowed to appear in
messages other than as CRLF; this use is also shown here.
Other differences in syntax and semantics are noted in the following
sections.
<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>. Miscellaneous Obsolete Tokens</span>
These syntactic elements are used elsewhere in the obsolete syntax or
in the main syntax. Bare CR, bare LF, and NUL are added to obs-qp,
obs-body, and obs-unstruct. US-ASCII control characters are added to
obs-qp, obs-unstruct, obs-ctext, and obs-qtext. The period character
is added to obs-phrase. The obs-phrase-list provides for a
(potentially empty) comma-separated list of phrases that may include
"null" elements. That is, there could be two or more commas in such
a list with nothing in between them, or commas at the beginning or
end of the list.
Note: The "period" (or "full stop") character (".") in obs-phrase
is not a form that was allowed in earlier versions of this or any
other specification. Period (nor any other character from
specials) was not allowed in phrase because it introduced a
parsing difficulty distinguishing between phrases and portions of
an addr-spec (see <a href="#section-4.4">section 4.4</a>). It appears here because the
period character is currently used in many messages in the
display-name portion of addresses, especially for initials in
names, and therefore must be interpreted properly.
obs-NO-WS-CTL = %d1-8 / ; US-ASCII control
%d11 / ; characters that do not
%d12 / ; include the carriage
%d14-31 / ; return, line feed, and
%d127 ; white space characters
obs-ctext = obs-NO-WS-CTL
obs-qtext = obs-NO-WS-CTL
obs-utext = %d0 / obs-NO-WS-CTL / VCHAR
obs-qp = "\" (%d0 / obs-NO-WS-CTL / LF / CR)
obs-body = *((*LF *CR *((%d0 / text) *LF *CR)) / CRLF)
obs-unstruct = *((*LF *CR *(obs-utext *LF *CR)) / FWS)
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obs-phrase = word *(word / "." / CFWS)
obs-phrase-list = [phrase / CFWS] *("," [phrase / CFWS])
Bare CR and bare LF appear in messages with two different meanings.
In many cases, bare CR or bare LF are used improperly instead of CRLF
to indicate line separators. In other cases, bare CR and bare LF are
used simply as US-ASCII control characters with their traditional
ASCII meanings.
<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>. Obsolete Folding White Space</span>
In the obsolete syntax, any amount of folding white space MAY be
inserted where the obs-FWS rule is allowed. This creates the
possibility of having two consecutive "folds" in a line, and
therefore the possibility that a line which makes up a folded header
field could be composed entirely of white space.
obs-FWS = 1*WSP *(CRLF 1*WSP)
<span class="h3"><a class="selflink" id="section-4.3" href="#section-4.3">4.3</a>. Obsolete Date and Time</span>
The syntax for the obsolete date format allows a 2 digit year in the
date field and allows for a list of alphabetic time zone specifiers
that were used in earlier versions of this specification. It also
permits comments and folding white space between many of the tokens.
obs-day-of-week = [CFWS] day-name [CFWS]
obs-day = [CFWS] 1*2DIGIT [CFWS]
obs-year = [CFWS] 2*DIGIT [CFWS]
obs-hour = [CFWS] 2DIGIT [CFWS]
obs-minute = [CFWS] 2DIGIT [CFWS]
obs-second = [CFWS] 2DIGIT [CFWS]
obs-zone = "UT" / "GMT" / ; Universal Time
; North American UT
; offsets
"EST" / "EDT" / ; Eastern: - 5/ - 4
"CST" / "CDT" / ; Central: - 6/ - 5
"MST" / "MDT" / ; Mountain: - 7/ - 6
"PST" / "PDT" / ; Pacific: - 8/ - 7
;
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%d65-73 / ; Military zones - "A"
%d75-90 / ; through "I" and "K"
%d97-105 / ; through "Z", both
%d107-122 ; upper and lower case
Where a two or three digit year occurs in a date, the year is to be
interpreted as follows: If a two digit year is encountered whose
value is between 00 and 49, the year is interpreted by adding 2000,
ending up with a value between 2000 and 2049. If a two digit year is
encountered with a value between 50 and 99, or any three digit year
is encountered, the year is interpreted by adding 1900.
In the obsolete time zone, "UT" and "GMT" are indications of
"Universal Time" and "Greenwich Mean Time", respectively, and are
both semantically identical to "+0000".
The remaining three character zones are the US time zones. The first
letter, "E", "C", "M", or "P" stands for "Eastern", "Central",
"Mountain", and "Pacific". The second letter is either "S" for
"Standard" time, or "D" for "Daylight Savings" (or summer) time.
Their interpretations are as follows:
EDT is semantically equivalent to -0400
EST is semantically equivalent to -0500
CDT is semantically equivalent to -0500
CST is semantically equivalent to -0600
MDT is semantically equivalent to -0600
MST is semantically equivalent to -0700
PDT is semantically equivalent to -0700
PST is semantically equivalent to -0800
The 1 character military time zones were defined in a non-standard
way in [<a href="./rfc0822" title=""Standard for the format of ARPA Internet text messages"">RFC0822</a>] and are therefore unpredictable in their meaning.
The original definitions of the military zones "A" through "I" are
equivalent to "+0100" through "+0900", respectively; "K", "L", and
"M" are equivalent to "+1000", "+1100", and "+1200", respectively;
"N" through "Y" are equivalent to "-0100" through "-1200".
respectively; and "Z" is equivalent to "+0000". However, because of
the error in [<a href="./rfc0822" title=""Standard for the format of ARPA Internet text messages"">RFC0822</a>], they SHOULD all be considered equivalent to
"-0000" unless there is out-of-band information confirming their
meaning.
Other multi-character (usually between 3 and 5) alphabetic time zones
have been used in Internet messages. Any such time zone whose
meaning is not known SHOULD be considered equivalent to "-0000"
unless there is out-of-band information confirming their meaning.
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<span class="h3"><a class="selflink" id="section-4.4" href="#section-4.4">4.4</a>. Obsolete Addressing</span>
There are four primary differences in addressing. First, mailbox
addresses were allowed to have a route portion before the addr-spec
when enclosed in "<" and ">". The route is simply a comma-separated
list of domain names, each preceded by "@", and the list terminated
by a colon. Second, CFWS were allowed between the period-separated
elements of local-part and domain (i.e., dot-atom was not used). In
addition, local-part is allowed to contain quoted-string in addition
to just atom. Third, mailbox-list and address-list were allowed to
have "null" members. That is, there could be two or more commas in
such a list with nothing in between them, or commas at the beginning
or end of the list. Finally, US-ASCII control characters and quoted-
pairs were allowed in domain literals and are added here.
obs-angle-addr = [CFWS] "<" obs-route addr-spec ">" [CFWS]
obs-route = obs-domain-list ":"
obs-domain-list = *(CFWS / ",") "@" domain
*("," [CFWS] ["@" domain])
obs-mbox-list = *([CFWS] ",") mailbox *("," [mailbox / CFWS])
obs-addr-list = *([CFWS] ",") address *("," [address / CFWS])
obs-group-list = 1*([CFWS] ",") [CFWS]
obs-local-part = word *("." word)
obs-domain = atom *("." atom)
obs-dtext = obs-NO-WS-CTL / quoted-pair
When interpreting addresses, the route portion SHOULD be ignored.
<span class="h3"><a class="selflink" id="section-4.5" href="#section-4.5">4.5</a>. Obsolete Header Fields</span>
Syntactically, the primary difference in the obsolete field syntax is
that it allows multiple occurrences of any of the fields and they may
occur in any order. Also, any amount of white space is allowed
before the ":" at the end of the field name.
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obs-fields = *(obs-return /
obs-received /
obs-orig-date /
obs-from /
obs-sender /
obs-reply-to /
obs-to /
obs-cc /
obs-bcc /
obs-message-id /
obs-in-reply-to /
obs-references /
obs-subject /
obs-comments /
obs-keywords /
obs-resent-date /
obs-resent-from /
obs-resent-send /
obs-resent-rply /
obs-resent-to /
obs-resent-cc /
obs-resent-bcc /
obs-resent-mid /
obs-optional)
Except for destination address fields (described in <a href="#section-4.5.3">section 4.5.3</a>),
the interpretation of multiple occurrences of fields is unspecified.
Also, the interpretation of trace fields and resent fields that do
not occur in blocks prepended to the message is unspecified as well.
Unless otherwise noted in the following sections, interpretation of
other fields is identical to the interpretation of their non-obsolete
counterparts in <a href="#section-3">section 3</a>.
<span class="h4"><a class="selflink" id="section-4.5.1" href="#section-4.5.1">4.5.1</a>. Obsolete Origination Date Field</span>
obs-orig-date = "Date" *WSP ":" date-time CRLF
<span class="h4"><a class="selflink" id="section-4.5.2" href="#section-4.5.2">4.5.2</a>. Obsolete Originator Fields</span>
obs-from = "From" *WSP ":" mailbox-list CRLF
obs-sender = "Sender" *WSP ":" mailbox CRLF
obs-reply-to = "Reply-To" *WSP ":" address-list CRLF
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<span class="h4"><a class="selflink" id="section-4.5.3" href="#section-4.5.3">4.5.3</a>. Obsolete Destination Address Fields</span>
obs-to = "To" *WSP ":" address-list CRLF
obs-cc = "Cc" *WSP ":" address-list CRLF
obs-bcc = "Bcc" *WSP ":"
(address-list / (*([CFWS] ",") [CFWS])) CRLF
When multiple occurrences of destination address fields occur in a
message, they SHOULD be treated as if the address list in the first
occurrence of the field is combined with the address lists of the
subsequent occurrences by adding a comma and concatenating.
<span class="h4"><a class="selflink" id="section-4.5.4" href="#section-4.5.4">4.5.4</a>. Obsolete Identification Fields</span>
The obsolete "In-Reply-To:" and "References:" fields differ from the
current syntax in that they allow phrase (words or quoted strings) to
appear. The obsolete forms of the left and right sides of msg-id
allow interspersed CFWS, making them syntactically identical to
local-part and domain, respectively.
obs-message-id = "Message-ID" *WSP ":" msg-id CRLF
obs-in-reply-to = "In-Reply-To" *WSP ":" *(phrase / msg-id) CRLF
obs-references = "References" *WSP ":" *(phrase / msg-id) CRLF
obs-id-left = local-part
obs-id-right = domain
For purposes of interpretation, the phrases in the "In-Reply-To:" and
"References:" fields are ignored.
Semantically, none of the optional CFWS in the local-part and the
domain is part of the obs-id-left and obs-id-right, respectively.
<span class="h4"><a class="selflink" id="section-4.5.5" href="#section-4.5.5">4.5.5</a>. Obsolete Informational Fields</span>
obs-subject = "Subject" *WSP ":" unstructured CRLF
obs-comments = "Comments" *WSP ":" unstructured CRLF
obs-keywords = "Keywords" *WSP ":" obs-phrase-list CRLF
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<span class="h4"><a class="selflink" id="section-4.5.6" href="#section-4.5.6">4.5.6</a>. Obsolete Resent Fields</span>
The obsolete syntax adds a "Resent-Reply-To:" field, which consists
of the field name, the optional comments and folding white space, the
colon, and a comma separated list of addresses.
obs-resent-from = "Resent-From" *WSP ":" mailbox-list CRLF
obs-resent-send = "Resent-Sender" *WSP ":" mailbox CRLF
obs-resent-date = "Resent-Date" *WSP ":" date-time CRLF
obs-resent-to = "Resent-To" *WSP ":" address-list CRLF
obs-resent-cc = "Resent-Cc" *WSP ":" address-list CRLF
obs-resent-bcc = "Resent-Bcc" *WSP ":"
(address-list / (*([CFWS] ",") [CFWS])) CRLF
obs-resent-mid = "Resent-Message-ID" *WSP ":" msg-id CRLF
obs-resent-rply = "Resent-Reply-To" *WSP ":" address-list CRLF
As with other resent fields, the "Resent-Reply-To:" field is to be
treated as trace information only.
<span class="h4"><a class="selflink" id="section-4.5.7" href="#section-4.5.7">4.5.7</a>. Obsolete Trace Fields</span>
The obs-return and obs-received are again given here as template
definitions, just as return and received are in <a href="#section-3">section 3</a>. Their
full syntax is given in [<a href="./rfc5321" title=""Simple Mail Transfer Protocol"">RFC5321</a>].
obs-return = "Return-Path" *WSP ":" path CRLF
obs-received = "Received" *WSP ":" *received-token CRLF
<span class="h4"><a class="selflink" id="section-4.5.8" href="#section-4.5.8">4.5.8</a>. Obsolete optional fields</span>
obs-optional = field-name *WSP ":" unstructured CRLF
<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>. Security Considerations</span>
Care needs to be taken when displaying messages on a terminal or
terminal emulator. Powerful terminals may act on escape sequences
and other combinations of US-ASCII control characters with a variety
of consequences. They can remap the keyboard or permit other
modifications to the terminal that could lead to denial of service or
even damaged data. They can trigger (sometimes programmable)
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answerback messages that can allow a message to cause commands to be
issued on the recipient's behalf. They can also affect the operation
of terminal attached devices such as printers. Message viewers may
wish to strip potentially dangerous terminal escape sequences from
the message prior to display. However, other escape sequences appear
in messages for useful purposes (cf. [<a href="#ref-ISO.2022.1994" title=""Information technology - Character code structure and extension techniques"">ISO.2022.1994</a>], [<a href="./rfc2045" title=""Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies"">RFC2045</a>],
[<a href="./rfc2046" title=""Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types"">RFC2046</a>], [<a href="./rfc2047" title=""MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text"">RFC2047</a>], [<a href="./rfc2049" title=""Multipurpose Internet Mail Extensions (MIME) Part Five: Conformance Criteria and Examples"">RFC2049</a>], [<a href="./rfc4288" title=""Media Type Specifications and Registration Procedures"">RFC4288</a>], [<a href="./rfc4289" title=""Multipurpose Internet Mail Extensions (MIME) Part Four: Registration Procedures"">RFC4289</a>]) and therefore
should not be stripped indiscriminately.
Transmission of non-text objects in messages raises additional
security issues. These issues are discussed in [<a href="./rfc2045" title=""Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies"">RFC2045</a>], [<a href="./rfc2046" title=""Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types"">RFC2046</a>],
[<a href="./rfc2047" title=""MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text"">RFC2047</a>], [<a href="./rfc2049" title=""Multipurpose Internet Mail Extensions (MIME) Part Five: Conformance Criteria and Examples"">RFC2049</a>], [<a href="./rfc4288" title=""Media Type Specifications and Registration Procedures"">RFC4288</a>], and [<a href="./rfc4289" title=""Multipurpose Internet Mail Extensions (MIME) Part Four: Registration Procedures"">RFC4289</a>].
Many implementations use the "Bcc:" (blind carbon copy) field,
described in <a href="#section-3.6.3">section 3.6.3</a>, to facilitate sending messages to
recipients without revealing the addresses of one or more of the
addressees to the other recipients. Mishandling this use of "Bcc:"
may disclose confidential information that could eventually lead to
security problems through knowledge of even the existence of a
particular mail address. For example, if using the first method
described in <a href="#section-3.6.3">section 3.6.3</a>, where the "Bcc:" line is removed from the
message, blind recipients have no explicit indication that they have
been sent a blind copy, except insofar as their address does not
appear in the header section of a message. Because of this, one of
the blind addressees could potentially send a reply to all of the
shown recipients and accidentally reveal that the message went to the
blind recipient. When the second method from <a href="#section-3.6.3">section 3.6.3</a> is used,
the blind recipient's address appears in the "Bcc:" field of a
separate copy of the message. If the "Bcc:" field sent contains all
of the blind addressees, all of the "Bcc:" recipients will be seen by
each "Bcc:" recipient. Even if a separate message is sent to each
"Bcc:" recipient with only the individual's address, implementations
still need to be careful to process replies to the message as per
<a href="#section-3.6.3">section 3.6.3</a> so as not to accidentally reveal the blind recipient to
other recipients.
<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>. IANA Considerations</span>
This document updates the registrations that appeared in [<a href="./rfc4021" title=""Registration of Mail and MIME Header Fields"">RFC4021</a>]
that referred to the definitions in [<a href="./rfc2822" title=""Internet Message Format"">RFC2822</a>]. IANA has updated the
Permanent Message Header Field Repository with the following header
fields, in accordance with the procedures set out in [<a href="./rfc3864" title=""Registration Procedures for Message Header Fields"">RFC3864</a>].
Header field name: Date
Applicable protocol: Mail
Status: standard
Author/Change controller: IETF
Specification document(s): This document (<a href="#section-3.6.1">section 3.6.1</a>)
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Header field name: From
Applicable protocol: Mail
Status: standard
Author/Change controller: IETF
Specification document(s): This document (<a href="#section-3.6.2">section 3.6.2</a>)
Header field name: Sender
Applicable protocol: Mail
Status: standard
Author/Change controller: IETF
Specification document(s): This document (<a href="#section-3.6.2">section 3.6.2</a>)
Header field name: Reply-To
Applicable protocol: Mail
Status: standard
Author/Change controller: IETF
Specification document(s): This document (<a href="#section-3.6.2">section 3.6.2</a>)
Header field name: To
Applicable protocol: Mail
Status: standard
Author/Change controller: IETF
Specification document(s): This document (<a href="#section-3.6.3">section 3.6.3</a>)
Header field name: Cc
Applicable protocol: Mail
Status: standard
Author/Change controller: IETF
Specification document(s): This document (<a href="#section-3.6.3">section 3.6.3</a>)
Header field name: Bcc
Applicable protocol: Mail
Status: standard
Author/Change controller: IETF
Specification document(s): This document (<a href="#section-3.6.3">section 3.6.3</a>)
Header field name: Message-ID
Applicable protocol: Mail
Status: standard
Author/Change controller: IETF
Specification document(s): This document (<a href="#section-3.6.4">section 3.6.4</a>)
Header field name: In-Reply-To
Applicable protocol: Mail
Status: standard
Author/Change controller: IETF
Specification document(s): This document (<a href="#section-3.6.4">section 3.6.4</a>)
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Header field name: References
Applicable protocol: Mail
Status: standard
Author/Change controller: IETF
Specification document(s): This document (<a href="#section-3.6.4">section 3.6.4</a>)
Header field name: Subject
Applicable protocol: Mail
Status: standard
Author/Change controller: IETF
Specification document(s): This document (<a href="#section-3.6.5">section 3.6.5</a>)
Header field name: Comments
Applicable protocol: Mail
Status: standard
Author/Change controller: IETF
Specification document(s): This document (<a href="#section-3.6.5">section 3.6.5</a>)
Header field name: Keywords
Applicable protocol: Mail
Status: standard
Author/Change controller: IETF
Specification document(s): This document (<a href="#section-3.6.5">section 3.6.5</a>)
Header field name: Resent-Date
Applicable protocol: Mail
Status: standard
Author/Change controller: IETF
Specification document(s): This document (<a href="#section-3.6.6">section 3.6.6</a>)
Header field name: Resent-From
Applicable protocol: Mail
Status: standard
Author/Change controller: IETF
Specification document(s): This document (<a href="#section-3.6.6">section 3.6.6</a>)
Header field name: Resent-Sender
Applicable protocol: Mail
Status: standard
Author/Change controller: IETF
Specification document(s): This document (<a href="#section-3.6.6">section 3.6.6</a>)
Header field name: Resent-To
Applicable protocol: Mail
Status: standard
Author/Change controller: IETF
Specification document(s): This document (<a href="#section-3.6.6">section 3.6.6</a>)
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Header field name: Resent-Cc
Applicable protocol: Mail
Status: standard
Author/Change controller: IETF
Specification document(s): This document (<a href="#section-3.6.6">section 3.6.6</a>)
Header field name: Resent-Bcc
Applicable protocol: Mail
Status: standard
Author/Change controller: IETF
Specification document(s): This document (<a href="#section-3.6.6">section 3.6.6</a>)
Header field name: Resent-Reply-To
Applicable protocol: Mail
Status: obsolete
Author/Change controller: IETF
Specification document(s): This document (<a href="#section-4.5.6">section 4.5.6</a>)
Header field name: Resent-Message-ID
Applicable protocol: Mail
Status: standard
Author/Change controller: IETF
Specification document(s): This document (<a href="#section-3.6.6">section 3.6.6</a>)
Header field name: Return-Path
Applicable protocol: Mail
Status: standard
Author/Change controller: IETF
Specification document(s): This document (<a href="#section-3.6.7">section 3.6.7</a>)
Header field name: Received
Applicable protocol: Mail
Status: standard
Author/Change controller: IETF
Specification document(s): This document (<a href="#section-3.6.7">section 3.6.7</a>)
Related information: [<a href="./rfc5321" title=""Simple Mail Transfer Protocol"">RFC5321</a>]
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<span class="h2"><a class="selflink" id="appendix-A" href="#appendix-A">Appendix A</a>. Example Messages</span>
This section presents a selection of messages. These are intended to
assist in the implementation of this specification, but should not be
taken as normative; that is to say, although the examples in this
section were carefully reviewed, if there happens to be a conflict
between these examples and the syntax described in sections <a href="#section-3">3</a> and <a href="#section-4">4</a>
of this document, the syntax in those sections is to be taken as
correct.
In the text version of this document, messages in this section are
delimited between lines of "----". The "----" lines are not part of
the message itself.
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<span class="h3"><a class="selflink" id="appendix-A.1" href="#appendix-A.1">Appendix A.1</a>. Addressing Examples</span>
The following are examples of messages that might be sent between two
individuals.
<span class="h4"><a class="selflink" id="appendix-A.1.1" href="#appendix-A.1.1">Appendix A.1.1</a>. A Message from One Person to Another with Simple</span>
Addressing
This could be called a canonical message. It has a single author,
John Doe, a single recipient, Mary Smith, a subject, the date, a
message identifier, and a textual message in the body.
----
From: John Doe <jdoe@machine.example>
To: Mary Smith <mary@example.net>
Subject: Saying Hello
Date: Fri, 21 Nov 1997 09:55:06 -0600
Message-ID: <1234@local.machine.example>
This is a message just to say hello.
So, "Hello".
----
If John's secretary Michael actually sent the message, even though
John was the author and replies to this message should go back to
him, the sender field would be used:
----
From: John Doe <jdoe@machine.example>
Sender: Michael Jones <mjones@machine.example>
To: Mary Smith <mary@example.net>
Subject: Saying Hello
Date: Fri, 21 Nov 1997 09:55:06 -0600
Message-ID: <1234@local.machine.example>
This is a message just to say hello.
So, "Hello".
----
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<span class="h4"><a class="selflink" id="appendix-A.1.2" href="#appendix-A.1.2">Appendix A.1.2</a>. Different Types of Mailboxes</span>
This message includes multiple addresses in the destination fields
and also uses several different forms of addresses.
----
From: "Joe Q. Public" <john.q.public@example.com>
To: Mary Smith <mary@x.test>, [email protected], Who? <one@y.test>
Cc: <boss@nil.test>, "Giant; \"Big\" Box" <sysservices@example.net>
Date: Tue, 1 Jul 2003 10:52:37 +0200
Message-ID: <5678.21-Nov-1997@example.com>
Hi everyone.
----
Note that the display names for Joe Q. Public and Giant; "Big" Box
needed to be enclosed in double-quotes because the former contains
the period and the latter contains both semicolon and double-quote
characters (the double-quote characters appearing as quoted-pair
constructs). Conversely, the display name for Who? could appear
without them because the question mark is legal in an atom. Notice
also that [email protected] and [email protected] have no display names
associated with them at all, and [email protected] uses the simpler
address form without the angle brackets.
<span class="h4"><a class="selflink" id="appendix-A.1.3" href="#appendix-A.1.3">Appendix A.1.3</a>. Group Addresses</span>
----
From: Pete <pete@silly.example>
To: A Group:Ed Jones <c@a.test>,[email protected],John <jdoe@one.test>;
Cc: Undisclosed recipients:;
Date: Thu, 13 Feb 1969 23:32:54 -0330
Message-ID: <testabcd.1234@silly.example>
Testing.
----
In this message, the "To:" field has a single group recipient named
"A Group", which contains 3 addresses, and a "Cc:" field with an
empty group recipient named Undisclosed recipients.
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<span class="h3"><a class="selflink" id="appendix-A.2" href="#appendix-A.2">Appendix A.2</a>. Reply Messages</span>
The following is a series of three messages that make up a
conversation thread between John and Mary. John first sends a
message to Mary, Mary then replies to John's message, and then John
replies to Mary's reply message.
Note especially the "Message-ID:", "References:", and "In-Reply-To:"
fields in each message.
----
From: John Doe <jdoe@machine.example>
To: Mary Smith <mary@example.net>
Subject: Saying Hello
Date: Fri, 21 Nov 1997 09:55:06 -0600
Message-ID: <1234@local.machine.example>
This is a message just to say hello.
So, "Hello".
----
When sending replies, the Subject field is often retained, though
prepended with "Re: " as described in <a href="#section-3.6.5">section 3.6.5</a>.
----
From: Mary Smith <mary@example.net>
To: John Doe <jdoe@machine.example>
Reply-To: "Mary Smith: Personal Account" <smith@home.example>
Subject: Re: Saying Hello
Date: Fri, 21 Nov 1997 10:01:10 -0600
Message-ID: <3456@example.net>
In-Reply-To: <1234@local.machine.example>
References: <1234@local.machine.example>
This is a reply to your hello.
----
Note the "Reply-To:" field in the above message. When John replies
to Mary's message above, the reply should go to the address in the
"Reply-To:" field instead of the address in the "From:" field.
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----
To: "Mary Smith: Personal Account" <smith@home.example>
From: John Doe <jdoe@machine.example>
Subject: Re: Saying Hello
Date: Fri, 21 Nov 1997 11:00:00 -0600
Message-ID: <abcd.1234@local.machine.test>
In-Reply-To: <3456@example.net>
References: <1234@local.machine.example> <3456@example.net>
This is a reply to your reply.
----
<span class="h3"><a class="selflink" id="appendix-A.3" href="#appendix-A.3">Appendix A.3</a>. Resent Messages</span>
Start with the message that has been used as an example several
times:
----
From: John Doe <jdoe@machine.example>
To: Mary Smith <mary@example.net>
Subject: Saying Hello
Date: Fri, 21 Nov 1997 09:55:06 -0600
Message-ID: <1234@local.machine.example>
This is a message just to say hello.
So, "Hello".
----
Say that Mary, upon receiving this message, wishes to send a copy of
the message to Jane such that (a) the message would appear to have
come straight from John; (b) if Jane replies to the message, the
reply should go back to John; and (c) all of the original
information, like the date the message was originally sent to Mary,
the message identifier, and the original addressee, is preserved. In
this case, resent fields are prepended to the message:
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----
Resent-From: Mary Smith <mary@example.net>
Resent-To: Jane Brown <j-brown@other.example>
Resent-Date: Mon, 24 Nov 1997 14:22:01 -0800
Resent-Message-ID: <78910@example.net>
From: John Doe <jdoe@machine.example>
To: Mary Smith <mary@example.net>
Subject: Saying Hello
Date: Fri, 21 Nov 1997 09:55:06 -0600
Message-ID: <1234@local.machine.example>
This is a message just to say hello.
So, "Hello".
----
If Jane, in turn, wished to resend this message to another person,
she would prepend her own set of resent header fields to the above
and send that. (Note that for brevity, trace fields are not shown.)
<span class="h3"><a class="selflink" id="appendix-A.4" href="#appendix-A.4">Appendix A.4</a>. Messages with Trace Fields</span>
As messages are sent through the transport system as described in
[<a href="./rfc5321" title=""Simple Mail Transfer Protocol"">RFC5321</a>], trace fields are prepended to the message. The following
is an example of what those trace fields might look like. Note that
there is some folding white space in the first one since these lines
can be long.
----
Received: from x.y.test
by example.net
via TCP
with ESMTP
id ABC12345
for <mary@example.net>; 21 Nov 1997 10:05:43 -0600
Received: from node.example by x.y.test; 21 Nov 1997 10:01:22 -0600
From: John Doe <jdoe@node.example>
To: Mary Smith <mary@example.net>
Subject: Saying Hello
Date: Fri, 21 Nov 1997 09:55:06 -0600
Message-ID: <1234@local.node.example>
This is a message just to say hello.
So, "Hello".
----
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<span class="h3"><a class="selflink" id="appendix-A.5" href="#appendix-A.5">Appendix A.5</a>. White Space, Comments, and Other Oddities</span>
White space, including folding white space, and comments can be
inserted between many of the tokens of fields. Taking the example
from A.1.3, white space and comments can be inserted into all of the
fields.
----
From: Pete(A nice \) chap) <pete(his account)@silly.test(his host)>
To:A Group(Some people)
:Chris Jones <c@(Chris's host.)public.example>,
[email protected],
John <[email protected]> (my dear friend); (the end of the group)
Cc:(Empty list)(start)Hidden recipients :(nobody(that I know)) ;
Date: Thu,
13
Feb
1969
23:32
-0330 (Newfoundland Time)
Message-ID: <[email protected]>
Testing.
----
The above example is aesthetically displeasing, but perfectly legal.
Note particularly (1) the comments in the "From:" field (including
one that has a ")" character appearing as part of a quoted-pair); (2)
the white space absent after the ":" in the "To:" field as well as
the comment and folding white space after the group name, the special
character (".") in the comment in Chris Jones's address, and the
folding white space before and after "[email protected],"; (3) the
multiple and nested comments in the "Cc:" field as well as the
comment immediately following the ":" after "Cc"; (4) the folding
white space (but no comments except at the end) and the missing
seconds in the time of the date field; and (5) the white space before
(but not within) the identifier in the "Message-ID:" field.
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<span class="h3"><a class="selflink" id="appendix-A.6" href="#appendix-A.6">Appendix A.6</a>. Obsoleted Forms</span>
The following are examples of obsolete (that is, the "MUST NOT
generate") syntactic elements described in <a href="#section-4">section 4</a> of this
document.
<span class="h4"><a class="selflink" id="appendix-A.6.1" href="#appendix-A.6.1">Appendix A.6.1</a>. Obsolete Addressing</span>
Note in the example below the lack of quotes around Joe Q. Public,
the route that appears in the address for Mary Smith, the two commas
that appear in the "To:" field, and the spaces that appear around the
"." in the jdoe address.
----
From: Joe Q. Public <john.q.public@example.com>
To: Mary Smith <@node.test:[email protected]>, , jdoe@test . example
Date: Tue, 1 Jul 2003 10:52:37 +0200
Message-ID: <5678.21-Nov-1997@example.com>
Hi everyone.
----
<span class="h4"><a class="selflink" id="appendix-A.6.2" href="#appendix-A.6.2">Appendix A.6.2</a>. Obsolete Dates</span>
The following message uses an obsolete date format, including a non-
numeric time zone and a two digit year. Note that although the day-
of-week is missing, that is not specific to the obsolete syntax; it
is optional in the current syntax as well.
----
From: John Doe <jdoe@machine.example>
To: Mary Smith <mary@example.net>
Subject: Saying Hello
Date: 21 Nov 97 09:55:06 GMT
Message-ID: <1234@local.machine.example>
This is a message just to say hello.
So, "Hello".
----
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<span class="h4"><a class="selflink" id="appendix-A.6.3" href="#appendix-A.6.3">Appendix A.6.3</a>. Obsolete White Space and Comments</span>
White space and comments can appear between many more elements than
in the current syntax. Also, folding lines that are made up entirely
of white space are legal.
----
From : John Doe <jdoe@machine(comment). example>
To : Mary Smith
__
<mary@example.net>
Subject : Saying Hello
Date : Fri, 21 Nov 1997 09(comment): 55 : 06 -0600
Message-ID : <1234 @ local(blah) .machine .example>
This is a message just to say hello.
So, "Hello".
----
Note especially the second line of the "To:" field. It starts with
two space characters. (Note that "__" represent blank spaces.)
Therefore, it is considered part of the folding, as described in
<a href="#section-4.2">section 4.2</a>. Also, the comments and white space throughout
addresses, dates, and message identifiers are all part of the
obsolete syntax.
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<span class="h2"><a class="selflink" id="appendix-B" href="#appendix-B">Appendix B</a>. Differences from Earlier Specifications</span>
This appendix contains a list of changes that have been made in the
Internet Message Format from earlier specifications, specifically
[<a href="./rfc0822" title=""Standard for the format of ARPA Internet text messages"">RFC0822</a>], [<a href="./rfc1123" title=""Requirements for Internet Hosts - Application and Support"">RFC1123</a>], and [<a href="./rfc2822" title=""Internet Message Format"">RFC2822</a>]. Items marked with an asterisk
(*) below are items which appear in <a href="#section-4">section 4</a> of this document and
therefore can no longer be generated.
The following are the changes made from [<a href="./rfc0822" title=""Standard for the format of ARPA Internet text messages"">RFC0822</a>] and [<a href="./rfc1123" title=""Requirements for Internet Hosts - Application and Support"">RFC1123</a>] to
[<a href="./rfc2822" title=""Internet Message Format"">RFC2822</a>] that remain in this document:
1. Period allowed in obsolete form of phrase.
2. ABNF moved out of document, now in [<a href="./rfc5234" title=""Augmented BNF for Syntax Specifications: ABNF"">RFC5234</a>].
3. Four or more digits allowed for year.
4. Header field ordering (and lack thereof) made explicit.
5. Encrypted header field removed.
6. Specifically allow and give meaning to "-0000" time zone.
7. Folding white space is not allowed between every token.
8. Requirement for destinations removed.
9. Forwarding and resending redefined.
10. Extension header fields no longer specifically called out.
11. ASCII 0 (null) removed.*
12. Folding continuation lines cannot contain only white space.*
13. Free insertion of comments not allowed in date.*
14. Non-numeric time zones not allowed.*
15. Two digit years not allowed.*
16. Three digit years interpreted, but not allowed for generation.*
17. Routes in addresses not allowed.*
18. CFWS within local-parts and domains not allowed.*
19. Empty members of address lists not allowed.*
20. Folding white space between field name and colon not allowed.*
21. Comments between field name and colon not allowed.
22. Tightened syntax of in-reply-to and references.*
23. CFWS within msg-id not allowed.*
24. Tightened semantics of resent fields as informational only.
25. Resent-Reply-To not allowed.*
26. No multiple occurrences of fields (except resent and received).*
27. Free CR and LF not allowed.*
28. Line length limits specified.
29. Bcc more clearly specified.
<span class="grey">Resnick Standards Track [Page 52]</span><span id="page-53" ></span>
<span class="grey"><a href="./rfc5322">RFC 5322</a> Internet Message Format October 2008</span>
The following are changes from [<a href="./rfc2822" title=""Internet Message Format"">RFC2822</a>].
1. Assorted typographical/grammatical errors fixed and
clarifications made.
2. Changed "standard" to "document" or "specification" throughout.
3. Made distinction between "header field" and "header section".
4. Removed NO-WS-CTL from ctext, qtext, dtext, and unstructured.*
5. Moved discussion of specials to the "Atom" section. Moved text
to "Overall message syntax" section.
6. Simplified CFWS syntax.
7. Fixed unstructured syntax.
8. Changed date and time syntax to deal with white space in
obsolete date syntax.
9. Removed quoted-pair from domain literals and message
identifiers.*
10. Clarified that other specifications limit domain syntax.
11. Simplified "Bcc:" and "Resent-Bcc:" syntax.
12. Allowed optional-field to appear within trace information.
13. Removed no-fold-quote from msg-id. Clarified syntax
limitations.
14. Generalized "Received:" syntax to fix bugs and move definition
out of this document.
15. Simplified obs-qp. Fixed and simplified obs-utext (which now
only appears in the obsolete syntax). Removed obs-text and obs-
char, adding obs-body.
16. Fixed obsolete date syntax to allow for more (or less) comments
and white space.
17. Fixed all obsolete list syntax (obs-domain-list, obs-mbox-list,
obs-addr-list, obs-phrase-list, and the newly added obs-group-
list).
18. Fixed obs-reply-to syntax.
19. Fixed obs-bcc and obs-resent-bcc to allow empty lists.
20. Removed obs-path.
<span class="h2"><a class="selflink" id="appendix-C" href="#appendix-C">Appendix C</a>. Acknowledgements</span>
Many people contributed to this document. They included folks who
participated in the Detailed Revision and Update of Messaging
Standards (DRUMS) Working Group of the Internet Engineering Task
Force (IETF), the chair of DRUMS, the Area Directors of the IETF, and
people who simply sent their comments in via email. The editor is
deeply indebted to them all and thanks them sincerely. The below
list includes everyone who sent email concerning both this document
and [<a href="./rfc2822" title=""Internet Message Format"">RFC2822</a>]. Hopefully, everyone who contributed is named here:
+--------------------+----------------------+---------------------+
| Matti Aarnio | Tanaka Akira | Russ Allbery |
| Eric Allman | Harald Alvestrand | Ran Atkinson |
| Jos Backus | Bruce Balden | Dave Barr |
<span class="grey">Resnick Standards Track [Page 53]</span><span id="page-54" ></span>
<span class="grey"><a href="./rfc5322">RFC 5322</a> Internet Message Format October 2008</span>
| Alan Barrett | John Beck | J Robert von Behren |
| Jos den Bekker | D J Bernstein | James Berriman |
| Oliver Block | Norbert Bollow | Raj Bose |
| Antony Bowesman | Scott Bradner | Randy Bush |
| Tom Byrer | Bruce Campbell | Larry Campbell |
| W J Carpenter | Michael Chapman | Richard Clayton |
| Maurizio Codogno | Jim Conklin | R Kelley Cook |
| Nathan Coulter | Steve Coya | Mark Crispin |
| Dave Crocker | Matt Curtin | Michael D'Errico |
| Cyrus Daboo | Michael D Dean | Jutta Degener |
| Mark Delany | Steve Dorner | Harold A Driscoll |
| Michael Elkins | Frank Ellerman | Robert Elz |
| Johnny Eriksson | Erik E Fair | Roger Fajman |
| Patrik Faltstrom | Claus Andre Faerber | Barry Finkel |
| Erik Forsberg | Chuck Foster | Paul Fox |
| Klaus M Frank | Ned Freed | Jochen Friedrich |
| Randall C Gellens | Sukvinder Singh Gill | Tim Goodwin |
| Philip Guenther | Arnt Gulbrandsen | Eric A Hall |
| Tony Hansen | John Hawkinson | Philip Hazel |
| Kai Henningsen | Robert Herriot | Paul Hethmon |
| Jim Hill | Alfred Hoenes | Paul E Hoffman |
| Steve Hole | Kari Hurtta | Marco S Hyman |
| Ofer Inbar | Olle Jarnefors | Kevin Johnson |
| Sudish Joseph | Maynard Kang | Prabhat Keni |
| John C Klensin | Graham Klyne | Brad Knowles |
| Shuhei Kobayashi | Peter Koch | Dan Kohn |
| Christian Kuhtz | Anand Kumria | Steen Larsen |
| Eliot Lear | Barry Leiba | Jay Levitt |
| Bruce Lilly | Lars-Johan Liman | Charles Lindsey |
| Pete Loshin | Simon Lyall | Bill Manning |
| John Martin | Mark Martinec | Larry Masinter |
| Denis McKeon | William P McQuillan | Alexey Melnikov |
| Perry E Metzger | Steven Miller | S Moonesamy |
| Keith Moore | John Gardiner Myers | Chris Newman |
| John W Noerenberg | Eric Norman | Mike O'Dell |
| Larry Osterman | Paul Overell | Jacob Palme |
| Michael A Patton | Uzi Paz | Michael A Quinlan |
| Robert Rapplean | Eric S Raymond | Sam Roberts |
| Hugh Sasse | Bart Schaefer | Tom Scola |
| Wolfgang Segmuller | Nick Shelness | John Stanley |
| Einar Stefferud | Jeff Stephenson | Bernard Stern |
| Peter Sylvester | Mark Symons | Eric Thomas |
| Lee Thompson | Karel De Vriendt | Matthew Wall |
| Rolf Weber | Brent B Welch | Dan Wing |
| Jack De Winter | Gregory J Woodhouse | Greg A Woods |
| Kazu Yamamoto | Alain Zahm | Jamie Zawinski |
| Timothy S Zurcher | | |
+--------------------+----------------------+---------------------+
<span class="grey">Resnick Standards Track [Page 54]</span><span id="page-55" ></span>
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<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>. References</span>
<span class="h3"><a class="selflink" id="section-7.1" href="#section-7.1">7.1</a>. Normative References</span>
[<a id="ref-ANSI.X3-4.1986">ANSI.X3-4.1986</a>] American National Standards Institute, "Coded
Character Set - 7-bit American Standard Code for
Information Interchange", ANSI X3.4, 1986.
[<a id="ref-RFC1034">RFC1034</a>] Mockapetris, P., "Domain names - concepts and
facilities", STD 13, <a href="./rfc1034">RFC 1034</a>, November 1987.
[<a id="ref-RFC1035">RFC1035</a>] Mockapetris, P., "Domain names - implementation and
specification", STD 13, <a href="./rfc1035">RFC 1035</a>, November 1987.
[<a id="ref-RFC1123">RFC1123</a>] Braden, R., "Requirements for Internet Hosts -
Application and Support", STD 3, <a href="./rfc1123">RFC 1123</a>,
October 1989.
[<a id="ref-RFC2119">RFC2119</a>] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>, March 1997.
[<a id="ref-RFC5234">RFC5234</a>] Crocker, D. and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", STD 68, <a href="./rfc5234">RFC 5234</a>,
January 2008.
<span class="h3"><a class="selflink" id="section-7.2" href="#section-7.2">7.2</a>. Informative References</span>
[<a id="ref-RFC0822">RFC0822</a>] Crocker, D., "Standard for the format of ARPA
Internet text messages", STD 11, <a href="./rfc822">RFC 822</a>,
August 1982.
[<a id="ref-RFC1305">RFC1305</a>] Mills, D., "Network Time Protocol (Version 3)
Specification, Implementation", <a href="./rfc1305">RFC 1305</a>,
March 1992.
[<a id="ref-ISO.2022.1994">ISO.2022.1994</a>] International Organization for Standardization,
"Information technology - Character code structure
and extension techniques", ISO Standard 2022, 1994.
[<a id="ref-RFC2045">RFC2045</a>] Freed, N. and N. Borenstein, "Multipurpose Internet
Mail Extensions (MIME) Part One: Format of Internet
Message Bodies", <a href="./rfc2045">RFC 2045</a>, November 1996.
[<a id="ref-RFC2046">RFC2046</a>] Freed, N. and N. Borenstein, "Multipurpose Internet
Mail Extensions (MIME) Part Two: Media Types",
<a href="./rfc2046">RFC 2046</a>, November 1996.
<span class="grey">Resnick Standards Track [Page 55]</span><span id="page-56" ></span>
<span class="grey"><a href="./rfc5322">RFC 5322</a> Internet Message Format October 2008</span>
[<a id="ref-RFC2047">RFC2047</a>] Moore, K., "MIME (Multipurpose Internet Mail
Extensions) Part Three: Message Header Extensions
for Non-ASCII Text", <a href="./rfc2047">RFC 2047</a>, November 1996.
[<a id="ref-RFC2049">RFC2049</a>] Freed, N. and N. Borenstein, "Multipurpose Internet
Mail Extensions (MIME) Part Five: Conformance
Criteria and Examples", <a href="./rfc2049">RFC 2049</a>, November 1996.
[<a id="ref-RFC2822">RFC2822</a>] Resnick, P., "Internet Message Format", <a href="./rfc2822">RFC 2822</a>,
April 2001.
[<a id="ref-RFC3864">RFC3864</a>] Klyne, G., Nottingham, M., and J. Mogul,
"Registration Procedures for Message Header
Fields", <a href="https://www.rfc-editor.org/bcp/bcp90">BCP 90</a>, <a href="./rfc3864">RFC 3864</a>, September 2004.
[<a id="ref-RFC4021">RFC4021</a>] Klyne, G. and J. Palme, "Registration of Mail and
MIME Header Fields", <a href="./rfc4021">RFC 4021</a>, March 2005.
[<a id="ref-RFC4288">RFC4288</a>] Freed, N. and J. Klensin, "Media Type
Specifications and Registration Procedures",
<a href="https://www.rfc-editor.org/bcp/bcp13">BCP 13</a>, <a href="./rfc4288">RFC 4288</a>, December 2005.
[<a id="ref-RFC4289">RFC4289</a>] Freed, N. and J. Klensin, "Multipurpose Internet
Mail Extensions (MIME) Part Four: Registration
Procedures", <a href="https://www.rfc-editor.org/bcp/bcp13">BCP 13</a>, <a href="./rfc4289">RFC 4289</a>, December 2005.
[<a id="ref-RFC5321">RFC5321</a>] Klensin, J., "Simple Mail Transfer Protocol",
<a href="./rfc5321">RFC 5321</a>, October 2008.
Author's Address
Peter W. Resnick (editor)
Qualcomm Incorporated
5775 Morehouse Drive
San Diego, CA 92121-1714
US
Phone: +1 858 651 4478
EMail: [email protected]
URI: <a href="http://www.qualcomm.com/~presnick/">http://www.qualcomm.com/~presnick/</a>
<span class="grey">Resnick Standards Track [Page 56]</span><span id="page-57" ></span>
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Full Copyright Statement
Copyright (C) The IETF Trust (2008).
This document is subject to the rights, licenses and restrictions
contained in <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a>, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and <a href="https://www.rfc-editor.org/bcp/bcp79">BCP 79</a>.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
<a href="http://www.ietf.org/ipr">http://www.ietf.org/ipr</a>.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
[email protected].
Resnick Standards Track [Page 57]
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