Internet DRAFT - draft-ietf-emailcore-rfc5322bis
draft-ietf-emailcore-rfc5322bis
Network Working Group P. Resnick, Ed.
Internet-Draft Episteme
Obsoletes: 5322 (if approved) 27 January 2024
Updates: 3864, 4021 (if approved)
Intended status: Standards Track
Expires: 30 July 2024
Internet Message Format
draft-ietf-emailcore-rfc5322bis-10
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) 5322, itself a revision of
Request For Comments (RFC) 2822, all of which supersede 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.
Status of This Memo
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Notational Conventions . . . . . . . . . . . . . . . . . 5
1.2.1. Requirements Notation . . . . . . . . . . . . . . . . 5
1.2.2. Syntactic Notation . . . . . . . . . . . . . . . . . 5
1.2.3. Structure of This Document . . . . . . . . . . . . . 5
2. Lexical Analysis of Messages . . . . . . . . . . . . . . . . 7
2.1. General Description . . . . . . . . . . . . . . . . . . . 7
2.1.1. Line Length Limits . . . . . . . . . . . . . . . . . 8
2.2. Header Fields . . . . . . . . . . . . . . . . . . . . . . 8
2.2.1. Unstructured Header Field Bodies . . . . . . . . . . 9
2.2.2. Structured Header Field Bodies . . . . . . . . . . . 9
2.2.3. Long Header Fields . . . . . . . . . . . . . . . . . 9
2.3. Body . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 10
3.2. Lexical Tokens . . . . . . . . . . . . . . . . . . . . . 11
3.2.1. Quoted characters . . . . . . . . . . . . . . . . . . 11
3.2.2. Folding White Space and Comments . . . . . . . . . . 12
3.2.3. Atom . . . . . . . . . . . . . . . . . . . . . . . . 13
3.2.4. Quoted Strings . . . . . . . . . . . . . . . . . . . 14
3.2.5. Miscellaneous Tokens . . . . . . . . . . . . . . . . 15
3.3. Date and Time Specification . . . . . . . . . . . . . . . 15
3.4. Address Specification . . . . . . . . . . . . . . . . . . 17
3.4.1. Addr-Spec Specification . . . . . . . . . . . . . . . 18
3.5. Overall Message Syntax . . . . . . . . . . . . . . . . . 19
3.6. Field Definitions . . . . . . . . . . . . . . . . . . . . 20
3.6.1. The Origination Date Field . . . . . . . . . . . . . 23
3.6.2. Originator Fields . . . . . . . . . . . . . . . . . . 23
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3.6.3. Destination Address Fields . . . . . . . . . . . . . 24
3.6.4. Identification Fields . . . . . . . . . . . . . . . . 26
3.6.5. Informational Fields . . . . . . . . . . . . . . . . 28
3.6.6. Resent Fields . . . . . . . . . . . . . . . . . . . . 29
3.6.7. Trace Fields . . . . . . . . . . . . . . . . . . . . 31
3.6.8. Optional Fields . . . . . . . . . . . . . . . . . . . 32
4. Obsolete Syntax . . . . . . . . . . . . . . . . . . . . . . . 32
4.1. Miscellaneous Obsolete Tokens . . . . . . . . . . . . . . 33
4.2. Obsolete Folding White Space . . . . . . . . . . . . . . 34
4.3. Obsolete Date and Time . . . . . . . . . . . . . . . . . 35
4.4. Obsolete Addressing . . . . . . . . . . . . . . . . . . . 36
4.5. Obsolete Header Fields . . . . . . . . . . . . . . . . . 37
4.5.1. Obsolete Origination Date Field . . . . . . . . . . . 38
4.5.2. Obsolete Originator Fields . . . . . . . . . . . . . 38
4.5.3. Obsolete Destination Address Fields . . . . . . . . . 38
4.5.4. Obsolete Identification Fields . . . . . . . . . . . 39
4.5.5. Obsolete Informational Fields . . . . . . . . . . . . 39
4.5.6. Obsolete Resent Fields . . . . . . . . . . . . . . . 40
4.5.7. Obsolete Trace Fields . . . . . . . . . . . . . . . . 40
4.5.8. Obsolete optional fields . . . . . . . . . . . . . . 40
5. Security Considerations . . . . . . . . . . . . . . . . . . . 41
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 41
6.1. Message Header Field Registry Changes . . . . . . . . . . 42
6.2. Updates to the Permanent Message Header Field Registry . 42
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 45
7.1. Normative References . . . . . . . . . . . . . . . . . . 45
7.2. Informative References . . . . . . . . . . . . . . . . . 46
Appendix A. Example Messages . . . . . . . . . . . . . . . . . . 48
A.1. Addressing Examples . . . . . . . . . . . . . . . . . . . 48
A.1.1. A Message from One Person to Another with Simple
Addressing . . . . . . . . . . . . . . . . . . . . . 48
A.1.2. Different Types of Mailboxes . . . . . . . . . . . . 49
A.1.3. Group Addresses . . . . . . . . . . . . . . . . . . . 49
A.2. Reply Messages . . . . . . . . . . . . . . . . . . . . . 49
A.3. Resent Messages . . . . . . . . . . . . . . . . . . . . . 50
A.4. Messages with Trace Fields . . . . . . . . . . . . . . . 51
A.5. White Space, Comments, and Other Oddities . . . . . . . . 52
A.6. Obsoleted Forms . . . . . . . . . . . . . . . . . . . . . 53
A.6.1. Obsolete Addressing . . . . . . . . . . . . . . . . . 53
A.6.2. Obsolete Dates . . . . . . . . . . . . . . . . . . . 53
A.6.3. Obsolete White Space and Comments . . . . . . . . . . 53
Appendix B. Differences from Earlier Specifications . . . . . . 54
Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 57
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 57
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1. Introduction
1.1. Scope
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 [RFC5322], itself a revision of [RFC2822], all of which
supersede [RFC0822], updating it to reflect current practice and
incorporating incremental changes that were specified in other RFCs
such as [RFC1123].
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 ([RFC2045], [RFC2046], [RFC2049]), 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
[I-D.ietf-emailcore-rfc5321bis] 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;
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| 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.
1.2. Notational Conventions
1.2.1. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[BCP14] RFC2119 RFC8174 when, and only when, they appear in all
capitals, as shown here.
1.2.2. Syntactic Notation
This specification uses the Augmented Backus-Naur Form (ABNF) [STD68]
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).
1.2.3. Structure of This Document
This document is divided into several sections.
This section, section 1, is a short introduction to the document.
Section 2 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.
Section 3 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
section 3 represents messages as they MUST be created. There are
also notes in section 3 to indicate if any of the options specified
in the syntax SHOULD be used over any of the others.
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Both sections 2 and 3 describe messages that are legal to generate
for purposes of this specification.
Section 4 of this document specifies an "obsolete" syntax. There are
references in section 3 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.
| Note: The dictionary definition of "obsolete" is "no longer in
| use or no longer useful". While this specification mandates
| that these syntactic elements no longer be generated, it also
| mandates that conformant parsers be able to support them. One
| reason for this latter requirement is that there are long-
| established sites on the Internet with mail archives that go
| back decades, archives with messages containing these elements.
| Similarly, many people have decades-old messages in their
| personal message stores, and for various reasons it is
| occasionally useful to not only read such messages but also
| resend or forward them to others. While these archives may
| only be mined occasionally, and messages from these personal
| stores rarely resent, they are nonetheless still in use, making
| "obsolete" the incorrect term to describe these elements.
|
| Later efforts to revise this specification contemplated
| changing the term to "legacy" or something that would more
| accurately describe the elements, but such a change was
| rejected due to fears that it would result in unnecessary
| confusion, especially among long-time users and implementers of
| the specification.
Section 5 details security considerations to take into account when
implementing this specification.
Appendix 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.
Appendix B lists the differences between this specification and
earlier specifications for Internet messages.
Appendix C contains acknowledgements.
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2. Lexical Analysis of Messages
2.1. General Description
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 [ANSI.X3-4.1986] 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
| ([RFC2045], [RFC2046], [RFC2047], [RFC2049], [BCP13]) and the
| Internationalized Email Headers specification ([RFC6532]), 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".)
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.
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2.1.1. Line Length Limits
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
[I-D.ietf-emailcore-rfc5321bis]) 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 [I-D.ietf-emailcore-rfc5321bis] 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.
2.2. Header Fields
Header fields are lines beginning with a field name, followed by a
colon (":", ASCII value 58), followed by a field body, and terminated
by CRLF. A field name MUST be composed of printable US-ASCII
characters except for space (SP, ASCII value 32) (i.e., characters
that have values between 33 and 126, inclusive) excluding colon. A
field body may be composed of printable US-ASCII characters,
including the space character, plus the horizontal tab (HTAB, ASCII
value 9) character. (Together, SP and HTAB are 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
section 2.2.3. All field bodies MUST conform to the syntax described
in sections 3 and 4 of this specification.
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2.2.1. Unstructured Header Field Bodies
Some field bodies in this specification are defined simply as
"unstructured" (which is specified in section 3.2.5 as any printable
US-ASCII characters, including the space character, plus the
horizontal tab character) 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 section 2.2.3).
2.2.2. Structured Header Field Bodies
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 3 and 4 of this specification. Many of these tokens are
allowed (according to their syntax) to be introduced or end with
comments (as described in section 3.2.2) as well as the white space
characters, and those white space characters are subject to "folding"
and "unfolding" as described in section 2.2.3. Semantic analysis of
structured field bodies is given along with their syntax.
2.2.3. Long Header Fields
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.
For example, the header field:
Subject: This is a test
can be represented as:
Subject: This
is a test
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
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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.
2.3. Body
The body of a message is simply lines of US-ASCII characters. The
only two limitations on the body are as follows:
* CR and LF MUST only occur together as CRLF; they MUST NOT appear
independently in the body.
* 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 ([RFC2045], [RFC2046],
| [RFC2049], [BCP13]), that extend (and limit) this specification
| to allow for different sorts of message bodies. Again, these
| mechanisms are beyond the scope of this document.
3. Syntax
3.1. Introduction
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 [STD68], Appendix B.1:
CR, LF, CRLF, HTAB, SP, WSP, DQUOTE, DIGIT, ALPHA, and VCHAR.
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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 section 4. 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, section 3 defines a grammar for the
generation of messages, with "obs-" elements that are to be ignored,
while section 4 adds grammar for the interpretation of messages.
3.2. Lexical Tokens
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 section 3.2.2 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.
3.2.1. Quoted characters
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.
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 section 4.
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3.2.2. Folding White Space and Comments
White space characters, including white space used in folding
(described in section 2.2.3), 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
section 3.2.4. 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 / ; VCHAR characters not including
%d42-91 / ; "(", ")", or "\"
%d93-126 /
obs-ctext
ccontent = ctext / quoted-pair / comment
comment = "(" *([FWS] ccontent) [FWS] ")"
CFWS = (1*([FWS] comment) [FWS]) / FWS
Throughout this specification, where FWS (the folding white space
token) appears, it indicates a place where folding, as discussed in
section 2.2.3, 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
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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.
3.2.3. Atom
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 VCHAR 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 / ; VCHAR 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.
3.2.4. Quoted Strings
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 / ; VCHAR characters not including
%d35-91 / ; "\" or the quote character
%d93-126 /
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.
3.2.5. Miscellaneous Tokens
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
3.3. Date and Time Specification
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 (at the
offset specified by the zone).
The date and time-of-day SHOULD express local time.
The zone specifies the offset from Coordinated Universal Time (UTC)
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 used to indicate that the time was
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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 [RFC3339]), and the last two digits
of the zone MUST be within the range 00 through 59.
3.4. Address Specification
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.
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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
("<" 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
section 3.4.1.
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.
3.4.1. Addr-Spec Specification
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 one or more of "."
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.
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| 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.,
| [STD13], [RFC1123], [I-D.ietf-emailcore-rfc5321bis]). It is
| therefore incumbent upon implementations to conform to the
| syntax of addresses for the context in which they are used.
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 / ; VCHAR characters not including
%d94-126 / ; "[", "]", or "\"
obs-dtext
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 [STD13] and [RFC1123]. 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 [I-D.ietf-emailcore-rfc5321bis]. 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.
3.5. Overall Message Syntax
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 section 2.1.1 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.
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message = (fields / obs-fields)
[CRLF body]
body = (*(*998text CRLF) *998text) / obs-body
text = %d1-9 / ; Characters excluding CR
%d11 / ; and LF
%d12 /
%d14-127
The header fields carry most of the semantic information and are
defined in section 3.6. The body is simply a series of lines of text
that are uninterpreted for the purposes of this specification.
3.6. Field Definitions
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 3.6.6 and 3.6.7 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
*(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.
+================+========+============+==========================+
| 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 |
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| | | | 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 | |
+----------------+--------+------------+--------------------------+
Table 1
The exact interpretation of each field is described in subsequent
sections.
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3.6.1. The Origination Date Field
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.)
3.6.2. Originator Fields
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 section
3.6.3 for more information on forming the destination addresses for a
reply.
3.6.3. Destination Address Fields
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. This is accomplished in
one of three ways:
1. The "Bcc:" field and its contents are not included in the message
that is sent, even though the message is sent to all of the
recipients in all of the destination ("To:", "Cc:", and "Bcc:")
fields.
2. The recipients in the "To:" and "Cc:" fields are sent a message
that does not include the "Bcc:" field or its contents, and the
recipients in the "Bcc:" field are sent a copy with an empty
"Bcc:" field.
3. The recipients in the "To:" and "Cc:" fields are sent a message
that does not include the "Bcc:" field or its contents, and the
recipients in the "Bcc:" field are each sent a personalized copy
of the message that includes a "Bcc:" field containing only their
own address.
Which method to use with "Bcc:" fields is implementation dependent,
but refer to the "Security Considerations" section (Section 5) 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
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| 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.
3.6.4. Identification Fields
Though listed as optional in the table (Table 1) in section 3.6,
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., [STD13],
| [RFC1123], [I-D.ietf-emailcore-rfc5321bis]). 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] "<" msg-id-internal ">" [CFWS]
msg-id-internal = id-left "@" id-right
id-left = dot-atom-text / obs-id-left
id-right = dot-atom-text / no-fold-literal / obs-id-right
no-fold-literal = "[" *dtext "]"
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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.
| 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 section 3.6.7)
| and resent fields (described in section 3.6.6). 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
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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.
| 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.
3.6.5. Informational Fields
The informational fields are all optional. The "Subject:" and
"Comments:" fields are unstructured fields as defined in section
2.2.1, 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.
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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
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.
3.6.6. Resent Fields
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 present. The "Resent-Message-ID:" field SHOULD be
present. "Resent-Sender:" SHOULD NOT be used if "Resent-Sender:"
would be identical to "Resent-From:".
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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
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).
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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.
The "Resent-Message-ID:" field provides a unique identifier for the
resent message.
3.6.7. Trace Fields
The trace fields form a group of header fields that normally includes
a "Return-Path:" field and/or one or more "Received:" fields or other
trace-optional fields that are defined by other specifications as
belonging within the trace fields grouping. 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. The trace-optional fields follow the syntax of section
3.6.8. Other specifications may provide for particular trace fields
and more precisely define their syntax and semantics. Those
specifications include [I-D.ietf-emailcore-rfc5321bis] for the forms
of "Received:" and "Return-path:" supplied by SMTP.
trace = [return]
*(received / trace-optional)
return = "Return-Path:" path CRLF
path = angle-addr / ([CFWS] "<" [CFWS] ">" [CFWS])
received = "Received:"
[1*received-token / CFWS] ";" date-time CRLF
received-token = word / angle-addr / addr-spec / domain
trace-optional = optional-field
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The trace fields provide an audit trail of message handling history.
They document actions taken as a message moves through the transport
system. A full discussion of the Internet mail use of the "Return-
Path:" and "Received:" trace fields is contained in
[I-D.ietf-emailcore-rfc5321bis]; other specifications such as
[I-D.ietf-emailcore-as] describe the use of other fields that are to
be interpreted as trace fields. In particular, the operational
behavior associated with any of the fields in the section of the
message (how, when, and whether they are added to or even removed
from messages as they are moved through the transport system) are not
described here. 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.
3.6.8. Optional Fields
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 VCHAR characters except 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.
optional-field = field-name ":" unstructured CRLF
field-name = 1*ftext ; Limit to 77 characters to
; stay within 78 char-per-
; line recommendation
ftext = %d33-57 / ; VCHAR characters not including
%d59-126 ; ":".
For the purposes of this specification, any optional field is
uninterpreted.
4. Obsolete Syntax
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 section 3,
they MUST be accepted and parsed by a conformant receiver. This
section documents many of these syntactic elements. (See the note in
Section 1.2.3 for an explanation of the term "obsolete".) Taking the
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grammar in section 3 and adding the definitions presented in this
section will result in the grammar to use for the interpretation of
messages.
| Note: There are 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 section 3.2.2 regarding lines composed entirely of
white space in comments and folding white space does not apply. See
the discussion of folding white space in section 4.2 below.
Finally, certain characters that were formerly allowed in messages
appear in this section. The NUL character (ASCII value 0) was once
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.
4.1. Miscellaneous Obsolete Tokens
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.
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| 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 section 4.4). 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 = *(%d0 / LF / CR / text)
obs-unstruct = *((*CR 1*(obs-utext / FWS)) / 1*LF) *CR
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.
4.2. Obsolete Folding White Space
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*([CRLF] WSP)
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4.3. Obsolete Date and Time
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 = [CFWS] (
"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
;
%d65-73 / ; Military zones - "A"
%d75-90 / ; through "I" and "K"
%d97-105 / ; through "Z", both
%d107-122 ; upper and lower case
) [CFWS]
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".
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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" (daylight saving 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 [RFC0822] 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 [RFC0822], 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.
4.4. Obsolete Addressing
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.
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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.
4.5. Obsolete Header Fields
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 section 4.5.3),
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 section 3.
4.5.1. Obsolete Origination Date Field
obs-orig-date = "Date" *WSP ":" date-time CRLF
4.5.2. Obsolete Originator Fields
obs-from = "From" *WSP ":" mailbox-list CRLF
obs-sender = "Sender" *WSP ":" mailbox CRLF
obs-reply-to = "Reply-To" *WSP ":" address-list CRLF
4.5.3. Obsolete Destination Address Fields
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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.
4.5.4. Obsolete Identification Fields
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.
4.5.5. Obsolete Informational Fields
obs-subject = "Subject" *WSP ":" unstructured CRLF
obs-comments = "Comments" *WSP ":" unstructured CRLF
obs-keywords = "Keywords" *WSP ":" obs-phrase-list CRLF
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4.5.6. Obsolete Resent Fields
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 informational only.
4.5.7. Obsolete Trace Fields
The obs-return and obs-received are again given here as template
definitions, just as return and received are in section 3. Their
full syntax is given in [I-D.ietf-emailcore-rfc5321bis].
obs-return = "Return-Path" *WSP ":" path CRLF
obs-received = "Received" *WSP ":"
[1*received-token / CFWS] [ ";" date-time ] CRLF
4.5.8. Obsolete optional fields
obs-optional = field-name *WSP ":" unstructured CRLF
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5. Security Considerations
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)
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. [ISO.2022.1994], [RFC2045],
[RFC2046], [RFC2047], [RFC2049], [BCP13]) and therefore should not be
stripped indiscriminately.
Transmission of non-text objects in messages raises additional
security issues. These issues are discussed in [RFC2045], [RFC2046],
[RFC2047], [RFC2049], [BCP13].
The "Bcc:" (blind carbon copy) field, described in section 3.6.3,
facilitates sending a message to a recipient without revealing that
recipient's address to other recipients. However, use of the field
itself requires care to avoid unintentional disclosures of addresses
or other confidential information. For example, if using the first
method described in section 3.6.3, where the "Bcc:" line is removed
from the message, "Bcc:" 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,
a "Bcc:" recipient might send a reply to all of the listed
recipients, accidentally revealing to those other recipients that the
message went to the "Bcc:" recipient. Even when the second or third
methods from section 3.6.3 are used, where a separate copy of the
message is sent to each "Bcc:" recipient, with only the individual's
address (or no address at all) appearing in the "Bcc:" field,
recipient implementations still need to be careful to process replies
to the message as per section 3.6.3 so as not to accidentally reveal
the "Bcc:" recipient to other recipients. Similar considerations
need to be given to the use of "Resent-Bcc:" field described in
section 3.6.6.
6. IANA Considerations
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6.1. Message Header Field Registry Changes
This document requests IANA to change the structures of the Permanent
Header Field Registry and the Provisional Header Field Registry,
adding an additional field called "Trace" to each entry in each
registries. The value of this field is binary, and should contain
either "yes" or " " (or however IANA normally encodes such binary
fields). It is only applicable if the "applicable protocol" is set
"mail" (in other cases, it should be set to " ") and indicates
whether the header field is to be treated as belonging to the block
of trace fields as defined in section 3.6.7. This document updates
[RFC3864] to add an additional field to the registration template for
the Permanent Header Field Registry and the Provisional Header Field
Repository as follows:
Trace: Whether or not this field belongs to the block of trace
fields as defined in section 3.6.7 of [Reference to this
document]. Only applicable if "Applicable Protocol" is set to
"mail".
IANA is requested to update the Permanent Header Field Registry and
the Provisional Header Field Registry to add this field to each
entry. This field should be empty for existing registrations unless
section 6.2 explicitly sets an entry to "yes".
6.2. Updates to the Permanent Message Header Field Registry
This document requests IANA to update the registrations that first
appeared in [RFC4021] and were subsequently updated by [RFC5322].
IANA is requested to update the Permanent Message Header Field
Registry with the following header fields, in accordance with the
procedures set out in [RFC3864] and section 6.1.
Header field name Date
Applicable protocol Mail
Status standard
Author/Change controller IETF
Specification document(s) This document (section 3.6.1)
Header field name From
Applicable protocol Mail
Status standard
Author/Change controller IETF
Specification document(s) This document (section 3.6.2)
Header field name Sender
Applicable protocol Mail
Status standard
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Author/Change controller IETF
Specification document(s) This document (section 3.6.2)
Header field name Reply-To
Applicable protocol Mail
Status standard
Author/Change controller IETF
Specification document(s) This document (section 3.6.2)
Header field name To
Applicable protocol Mail
Status standard
Author/Change controller IETF
Specification document(s) This document (section 3.6.3)
Header field name Cc
Applicable protocol Mail
Status standard
Author/Change controller IETF
Specification document(s) This document (section 3.6.3)
Header field name Bcc
Applicable protocol Mail
Status standard
Author/Change controller IETF
Specification document(s) This document (section 3.6.3)
Header field name Message-ID
Applicable protocol Mail
Status standard
Author/Change controller IETF
Specification document(s) This document (section 3.6.4)
Header field name In-Reply-To
Applicable protocol Mail
Status standard
Author/Change controller IETF
Specification document(s) This document (section 3.6.4)
Header field name References
Applicable protocol Mail
Status standard
Author/Change controller IETF
Specification document(s) This document (section 3.6.4)
Header field name Subject
Applicable protocol Mail
Status standard
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Author/Change controller IETF
Specification document(s) This document (section 3.6.5)
Header field name Comments
Applicable protocol Mail
Status standard
Author/Change controller IETF
Specification document(s) This document (section 3.6.5)
Header field name Keywords
Applicable protocol Mail
Status standard
Author/Change controller IETF
Specification document(s) This document (section 3.6.5)
Header field name Resent-Date
Applicable protocol Mail
Status standard
Author/Change controller IETF
Specification document(s) This document (section 3.6.6)
Header field name Resent-From
Applicable protocol Mail
Status standard
Author/Change controller IETF
Specification document(s) This document (section 3.6.6)
Header field name Resent-Sender
Applicable protocol Mail
Status standard
Author/Change controller IETF
Specification document(s) This document (section 3.6.6)
Header field name Resent-To
Applicable protocol Mail
Status standard
Author/Change controller IETF
Specification document(s) This document (section 3.6.6)
Header field name Resent-Cc
Applicable protocol Mail
Status standard
Author/Change controller IETF
Specification document(s) This document (section 3.6.6)
Header field name Resent-Bcc
Applicable protocol Mail
Status standard
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Author/Change controller IETF
Specification document(s) This document (section 3.6.6)
Header field name Resent-Reply-To
Applicable protocol Mail
Status obsolete
Author/Change controller IETF
Specification document(s) This document (section 4.5.6)
Header field name Resent-Message-ID
Applicable protocol Mail
Status standard
Author/Change controller IETF
Specification document(s) This document (section 3.6.6)
Header field name Return-Path
Applicable protocol Mail
Status standard
Author/Change controller IETF
Trace yes
Specification document(s) This document (section 3.6.7)
Related information [I-D.ietf-emailcore-rfc5321bis]
Header field name Received
Applicable protocol Mail
Status standard
Author/Change controller IETF
Trace yes
Specification document(s) This document (section 3.6.7)
Related information [I-D.ietf-emailcore-rfc5321bis]
7. References
7.1. Normative References
[ANSI.X3-4.1986]
American National Standards Institute, "Coded Character
Set - 7-bit American Standard Code for Information
Interchange", ANSI X3.4, 1986.
[BCP14] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, May 2017.
<https://www.rfc-editor.org/info/bcp14>
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[RFC1123] Braden, R., Ed., "Requirements for Internet Hosts -
Application and Support", STD 3, RFC 1123,
DOI 10.17487/RFC1123, October 1989,
<https://www.rfc-editor.org/info/rfc1123>.
[STD13] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
<https://www.rfc-editor.org/info/std13>
[STD68] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
<https://www.rfc-editor.org/info/std68>
7.2. Informative References
[BCP13] Freed, N. and J. Klensin, "Multipurpose Internet Mail
Extensions (MIME) Part Four: Registration Procedures",
BCP 13, RFC 4289, December 2005.
Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, January 2013.
<https://www.rfc-editor.org/info/bcp13>
[ISO.2022.1994]
International Organization for Standardization,
"Information technology - Character code structure and
extension techniques", ISO Standard 2022, 1994.
[RFC0822] Crocker, D., "STANDARD FOR THE FORMAT OF ARPA INTERNET
TEXT MESSAGES", STD 11, RFC 822, DOI 10.17487/RFC0822,
August 1982, <https://www.rfc-editor.org/info/rfc822>.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
<https://www.rfc-editor.org/info/rfc2045>.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
DOI 10.17487/RFC2046, November 1996,
<https://www.rfc-editor.org/info/rfc2046>.
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[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
Part Three: Message Header Extensions for Non-ASCII Text",
RFC 2047, DOI 10.17487/RFC2047, November 1996,
<https://www.rfc-editor.org/info/rfc2047>.
[RFC2049] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Five: Conformance Criteria and
Examples", RFC 2049, DOI 10.17487/RFC2049, November 1996,
<https://www.rfc-editor.org/info/rfc2049>.
[RFC2822] Resnick, P., Ed., "Internet Message Format", RFC 2822,
DOI 10.17487/RFC2822, April 2001,
<https://www.rfc-editor.org/info/rfc2822>.
[RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet:
Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
<https://www.rfc-editor.org/info/rfc3339>.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
DOI 10.17487/RFC3864, September 2004,
<https://www.rfc-editor.org/info/rfc3864>.
[RFC4021] Klyne, G. and J. Palme, "Registration of Mail and MIME
Header Fields", RFC 4021, DOI 10.17487/RFC4021, March
2005, <https://www.rfc-editor.org/info/rfc4021>.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
DOI 10.17487/RFC5322, October 2008,
<https://www.rfc-editor.org/info/rfc5322>.
[RFC6532] Yang, A., Steele, S., and N. Freed, "Internationalized
Email Headers", RFC 6532, DOI 10.17487/RFC6532, February
2012, <https://www.rfc-editor.org/info/rfc6532>.
[I-D.ietf-emailcore-rfc5321bis]
Klensin, J. C., "Simple Mail Transfer Protocol", Work in
Progress, Internet-Draft, draft-ietf-emailcore-rfc5321bis-
25, 24 January 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-
emailcore-rfc5321bis-25>.
[I-D.ietf-emailcore-as]
Klensin, J. C., Murchison, K., and E. Sam, "Applicability
Statement for IETF Core Email Protocols", Work in
Progress, Internet-Draft, draft-ietf-emailcore-as-09, 18
December 2023, <https://datatracker.ietf.org/doc/html/
draft-ietf-emailcore-as-09>.
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Appendix A. Example Messages
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 3 and 4
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.
A.1. Addressing Examples
The following are examples of messages that might be sent between two
individuals.
A.1.1. A Message from One Person to Another with Simple 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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A.1.2. Different Types of Mailboxes
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>, jdoe@example.org, 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 jdoe@example.org and boss@nil.test have no display names
associated with them at all, and jdoe@example.org uses the simpler
address form without the angle brackets.
A.1.3. Group Addresses
From: Pete <pete@silly.example>
To: A Group:Ed Jones <c@a.test>,joe@where.test,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.
A.2. Reply Messages
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.
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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 section 3.6.5.
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.
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.
A.3. Resent Messages
Start with the message that has been used as an example several
times:
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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:
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.)
A.4. Messages with Trace Fields
As messages are sent through the transport system as described in
[I-D.ietf-emailcore-rfc5321bis], 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.
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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".
A.5. White Space, Comments, and Other Oddities
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@silly.test(his host is silly)>
To:A Group(Some people)
:Chris Jones <c@public.example(.host of Chris)>,
joe@example.org,
John <jdoe@one.test> (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: <testabcd.1234@silly.test>
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 "joe@example.org,"; (3) the
multiple and nested comments in the "Cc:" field as well as the
comment immediately following the ":" after "Cc"; (4) the folding
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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.
A.6. Obsoleted Forms
The following are examples of obsolete (that is, the "MUST NOT
generate") syntactic elements described in section 4 of this
document.
A.6.1. Obsolete Addressing
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:mary@example.net>, , jdoe@test . example
Date: Tue, 1 Jul 2003 10:52:37 +0200
Message-ID: <5678.21-Nov-1997@example.com>
Hi everyone.
A.6.2. Obsolete Dates
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".
A.6.3. Obsolete White Space and Comments
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.
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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 "__" represents blank spaces.)
Therefore, it is considered part of the folding, as described in
section 4.2. Also, the comments and white space throughout
addresses, dates, and message identifiers are all part of the
obsolete syntax.
Appendix B. Differences from Earlier Specifications
This appendix contains a list of changes that have been made in the
Internet Message Format from earlier specifications, specifically
[RFC0822], [RFC1123], [RFC2822], and [RFC5322]. Items marked with an
asterisk (*) below are items which appear in section 4 of this
document and therefore can no longer be generated.
The following are the changes made from [RFC0822] and [RFC1123] to
[RFC2822]:
1. Period allowed in obsolete form of phrase.
2. ABNF moved out of document, now in [STD68].
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.*
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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.
The following are changes from [RFC2822] to [RFC5322]:
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 (erratum 373 (https://www.rfc-
editor.org/errata/eid373)).
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.
The following are changes from [RFC5322].
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1. Clarified addr-spec description (erratum 1766 (https://www.rfc-
editor.org/errata/eid1766)).
2. Fixed obs-unstruct to be more limited (erratum 1905
(https://www.rfc-editor.org/errata/eid1905)).*
3. Simplified obs-body (erratum 1906 (https://www.rfc-
editor.org/errata/eid1906)).*
4. Fixed obs-FWS to allow for a leading CRLF (erratum 1908
(https://www.rfc-editor.org/errata/eid1908)).*
5. Fixed comments within addresses in A.5 (errata 2515
(https://www.rfc-editor.org/errata/eid2515) and 2579
(https://www.rfc-editor.org/errata/eid2579)).
6. Fixed time zone description (erratum 2726 (https://www.rfc-
editor.org/errata/eid2726)).
7. Removed inappropriate uses of "sent" in 3.6.3, 3.6.6, and 5
(erratum 3048 (https://www.rfc-editor.org/errata/eid3048)).
8. Allow for CFWS in otherwise empty list of "Received:" field
tokens (erratum 3979 (https://www.rfc-editor.org/errata/
eid3979)).
9. Clarified that "printable" includes space, and replaced
"printable" with "VCHAR" in ABNF comments to clarify that it
doesn't include the space character (erratum 4692
(https://www.rfc-editor.org/errata/eid4692)).
10. Clarify midnight in time-of-day (erratum 5905 (https://www.rfc-
editor.org/errata/eid5905)).
11. Allow for date-time in obs-received (erratum 5867
(https://www.rfc-editor.org/errata/eid5867)).*
12. Separated out "msg-id-internal" in "msg-id" (2822bis list
discussion (https://mailarchive.ietf.org/arch/browse/ietf-
822/?gbt=1&index=g2ocdJIKBHBF8CrC3HxBcLR3-M0)).
13. Updated references to STD 13, STD 68, BCP 13, and BCP 14, and
reference for leap seconds to RFC 3339.
14. Fixed typo in daylight saving time in description of obs-zone.*
15. Added comment to field-name ABNF to remind that length can't be
greater than 77 (erratum 5918 (https://www.rfc-
editor.org/errata/eid5918)).
16. Clarified description in 4.5.6 as "trace information"".
17. Explained the use of the term "obsolete" in Section 1.2.3.
18. Updated syntactic and semantic descriptions of trace in 3.6.7 to
allow other fields that are treated as trace, and allow return-
path without any received. Moved optional-field syntax into
this section and out of the top portion of 3.6 to accommodate
this.
19. Updated Header Field Registries to include a "Trace" column.
20. Added optional CFWS around obs-zone (erratum 6639
(https://www.rfc-editor.org/errata/eid6639))
21. Change "Note:" paragraphs with normative text in sections 2.2.3,
3.4, and 3.6.6 to normal paragraphs and removed normative text
from "Note:" in section in 4.
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22. Rewrote "Bcc:" section (Section 3.6.3) to clarify and correct
error that allowed blank "Bcc:" line to go to "To:" and "Cc:"
recipients
23. Changed Security Considerations (Section 5) to account for
changes to "Bcc:" clarifications and added note about "Reent-
Bcc:"
// This last part to be removed before publication.
There are also 2 errata that were "Held For Document Update" that
have not been addressed:
1. Erratum 2950 (https://www.rfc-editor.org/errata/eid2950): As per
ticket #39 (https://trac.ietf.org/trac/emailcore/ticket/39),
there is no need to change the resent fields from "*" to "1*" in
3.6 as it doesn't really affect the syntax.
2. Erratum 3135 (https://www.rfc-editor.org/errata/eid3135): As per
ticket #35 (https://trac.ietf.org/trac/emailcore/ticket/35),
discussion of empty quoted-string will appear in
https://datatracker.ietf.org/doc/draft-ietf-emailcore-as/
Appendix C. Acknowledgements
Many people contributed to this document. They included participants
in and chairs of the Detailed Revision and Update of Messaging
Standards (DRUMS), Yet Another Mail (YAM), and Revision of Core Email
Specifications (EMAILCORE) Working Groups of the Internet Engineering
Task Force (IETF), the Area Directors of the IETF, reporters of
errata on earlier versions of this document, and people who simply
sent their comments in via email. The editor is deeply indebted to
them all and thanks them sincerely. (While the editor wishes to
thank them all by name as was done in the past, the list has gotten
so long to make including it here untenable. But the thanks is no
less heartfelt.)
Author's Address
Peter W. Resnick (editor)
Episteme Technology Consulting LLC
503 West Indiana Avenue
Urbana, IL 61801-4941
United States of America
Phone: +1 217 337 1905
Email: resnick@episteme.net
URI: https://www.episteme.net/
Resnick Expires 30 July 2024 [Page 57]
