rfc2822 RFC 2822 Internet Message Format
Network Working Group P. Resnick, Editor
Request for Comments: 2822 QUALCOMM Incorporated
Obsoletes: 822 April 2001
Category: Standards Track
Internet Message Format
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
This standard specifies a syntax for text messages that are sent
between computer users, within the framework of "electronic mail"
messages. This standard supersedes the one specified in 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.
Table of Contents
1. Introduction ............................................... 3
1.1. Scope .................................................... 3
1.2. Notational conventions ................................... 4
1.2.1. Requirements notation .................................. 4
1.2.2. Syntactic notation ..................................... 4
1.3. Structure of this document ............................... 4
2. Lexical Analysis of Messages ............................... 5
2.1. General Description ...................................... 5
2.1.1. Line Length Limits ..................................... 6
2.2. Header Fields ............................................ 7
2.2.1. Unstructured Header Field Bodies ....................... 7
2.2.2. Structured Header Field Bodies ......................... 7
2.2.3. Long Header Fields ..................................... 7
2.3. Body ..................................................... 8
3. Syntax ..................................................... 9
3.1. Introduction ............................................. 9
3.2. Lexical Tokens ........................................... 9
Resnick Standards Track [Page 1]
RFC 2822 Internet Message Format April 2001
3.2.1. Primitive Tokens ....................................... 9
3.2.2. Quoted characters ......................................10
3.2.3. Folding white space and comments .......................11
3.2.4. Atom ...................................................12
3.2.5. Quoted strings .........................................13
3.2.6. Miscellaneous tokens ...................................13
3.3. Date and Time Specification ..............................14
3.4. Address Specification ....................................15
3.4.1. Addr-spec specification ................................16
3.5 Overall message syntax ....................................17
3.6. Field definitions ........................................18
3.6.1. The origination date field .............................20
3.6.2. Originator fields ......................................21
3.6.3. Destination address fields .............................22
3.6.4. Identification fields ..................................23
3.6.5. Informational fields ...................................26
3.6.6. Resent fields ..........................................26
3.6.7. Trace fields ...........................................28
3.6.8. Optional fields ........................................29
4. Obsolete Syntax ............................................29
4.1. Miscellaneous obsolete tokens ............................30
4.2. Obsolete folding white space .............................31
4.3. Obsolete Date and Time ...................................31
4.4. Obsolete Addressing ......................................33
4.5. Obsolete header fields ...................................33
4.5.1. Obsolete origination date field ........................34
4.5.2. Obsolete originator fields .............................34
4.5.3. Obsolete destination address fields ....................34
4.5.4. Obsolete identification fields .........................35
4.5.5. Obsolete informational fields ..........................35
4.5.6. Obsolete resent fields .................................35
4.5.7. Obsolete trace fields ..................................36
4.5.8. Obsolete optional fields ...............................36
5. Security Considerations ....................................36
6. Bibliography ...............................................37
7. Editor's Address ...........................................38
8. Acknowledgements ...........................................39
Appendix A. Example messages ..................................41
A.1. Addressing examples ......................................41
A.1.1. A message from one person to another with simple
addressing .............................................41
A.1.2. Different types of mailboxes ...........................42
A.1.3. Group addresses ........................................43
A.2. Reply messages ...........................................43
A.3. Resent messages ..........................................44
A.4. Messages with trace fields ...............................46
A.5. White space, comments, and other oddities ................47
A.6. Obsoleted forms ..........................................47
Resnick Standards Track [Page 2]
RFC 2822 Internet Message Format April 2001
A.6.1. Obsolete addressing ....................................48
A.6.2. Obsolete dates .........................................48
A.6.3. Obsolete white space and comments ......................48
Appendix B. Differences from earlier standards ................49
Appendix C. Notices ...........................................50
Full Copyright Statement ......................................51
1. Introduction
1.1. Scope
This standard specifies a syntax for text messages that are sent
between computer users, within the framework of "electronic mail"
messages. This standard supersedes the one specified in Request For
Comments (RFC) 822, "Standard for the Format of ARPA Internet Text
Messages" [RFC822], updating it to reflect current practice and
incorporating incremental changes that were specified in other RFCs
[STD3].
This standard 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 standard.
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 [RFC2821] for a
discussion of the envelope.) The contents comprise the object to be
delivered to the recipient. This standard 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 standard 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 standard, local storage is
outside of the scope of this standard.
Resnick Standards Track [Page 3]
RFC 2822 Internet Message Format April 2001
Note: This standard 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 standard
does not specify an encoding of the characters for either transport
or storage; that is, it does not specify the number of bits used or
how those bits are specifically transferred over the wire or stored
on disk.
1.2. Notational conventions
1.2.1. Requirements notation
This document occasionally uses terms that appear in capital letters.
When the terms "MUST", "SHOULD", "RECOMMENDED", "MUST NOT", "SHOULD
NOT", and "MAY" appear capitalized, they are being used to indicate
particular requirements of this specification. A discussion of the
meanings of these terms appears in [RFC2119].
1.2.2. Syntactic notation
This standard uses the Augmented Backus-Naur Form (ABNF) notation
specified in [RFC2234] 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). See [RFC2234] for the full
description of the notation.
1.3. 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 describes the actual rules for the structure
of each part of a message (the syntax) as well as a description of
the parts and instructions on how they ought to be interpreted (the
semantics). This includes analysis of the syntax and semantics of
Resnick Standards Track [Page 4]
RFC 2822 Internet Message Format April 2001
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.
Both sections 2 and 3 describe messages that are legal to generate
for purposes of this standard.
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 revisions of this standard 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
standard. 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.
Section 5 details security considerations to take into account when
implementing this standard.
Section 6 is a bibliography of references in this document.
Section 7 contains the editor's address.
Section 8 contains acknowledgements.
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 standard and earlier
standards for Internet messages.
Appendix C has copyright and intellectual property notices.
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 standard is comprised of
characters with values in the range 1 through 127 and interpreted as
US-ASCII characters [ASCII]. For brevity, this document sometimes
refers to this range of characters as simply "US-ASCII characters".
Resnick Standards Track [Page 5]
RFC 2822 Internet Message Format April 2001
Note: This standard 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,
RFC2048, RFC2049], that extend this standard to allow for values
outside of that range. Discussion of those mechanisms is not within
the scope of this standard.
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
of the message") followed, optionally, by a body. The header is a
sequence of lines of characters with special syntax as defined in
this standard. The body is simply a sequence of characters that
follows the header and is separated from the header by an empty line
(i.e., a line with nothing preceding the CRLF).
2.1.1. Line Length Limits
There are two limits that this standard 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
which send, receive, or store Internet Message Format messages that
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 which (in compliance with the transport
requirements of [RFC2821]) do not accept messages containing more
than 1000 character 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 [RFC2821] if they actually cause
information to be lost). Again, even though this limitation is put on
messages, it is encumbant upon implementations which display messages
Resnick Standards Track [Page 6]
RFC 2822 Internet Message Format April 2001
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 composed of a field name, followed by a colon
(":"), followed by a field body, and terminated by CRLF. A field
name MUST be composed of printable US-ASCII characters (i.e.,
characters that have values between 33 and 126, inclusive), except
colon. A field body may be composed of any US-ASCII characters,
except for CR and LF. However, a field body may contain CRLF when
used in header "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 standard.
2.2.1. Unstructured Header Field Bodies
Some field bodies in this standard are defined simply as
"unstructured" (which is specified below as any US-ASCII characters,
except for CR and LF) 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 header "folding" and
"unfolding" as described in section 2.2.3).
2.2.2. Structured Header Field Bodies
Some field bodies in this standard have specific syntactical
structure 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 standard. Many of these tokens
are allowed (according to their syntax) to be introduced or end with
comments (as described in section 3.2.3) as well as the space (SP,
ASCII value 32) and horizontal tab (HTAB, ASCII value 9) characters
(together known as the white space characters, WSP), and those WSP
characters are subject to header "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
Resnick Standards Track [Page 7]
RFC 2822 Internet Message Format April 2001
that wherever this standard 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
Note: Though structured field bodies are defined in such a way that
folding can take place between many of the lexical tokens (and even
within some of the lexical tokens), folding SHOULD be limited to
placing the CRLF at higher-level syntactic breaks. For instance, if
a field body is defined as comma-separated values, it is recommended
that folding occur after the comma separating the structured items in
preference to other places where the field could be folded, even if
it is allowed elsewhere.
The process of moving from this folded multiple-line representation
of a header field to its single line representation is called
"unfolding". Unfolding is accomplished by simply removing any CRLF
that is immediately followed by WSP. Each header field should be
treated in its unfolded form for further syntactic and semantic
evaluation.
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 standards documents,
specifically the MIME documents [RFC2045, RFC2046, RFC2048, RFC2049]
that extend this standard to allow for different sorts of message
bodies. Again, these mechanisms are beyond the scope of this
document.
Resnick Standards Track [Page 8]
RFC 2822 Internet Message Format April 2001
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 standard
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. Primitive tokens that are used but otherwise unspecified
come from [RFC2234].
In some of the definitions, there will be nonterminals 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
generation of messages, with "obs-" elements that are to be ignored,
while section 4 adds grammar for 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 standard 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 3.2.3 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, the 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. Primitive Tokens
The following are primitive tokens referred to elsewhere in this
standard, but not otherwise defined in [RFC2234]. Some of them will
not appear anywhere else in the syntax, but they are convenient to
refer to in other parts of this document.
Resnick Standards Track [Page 9]
RFC 2822 Internet Message Format April 2001
Note: The "specials" below are just such an example. Though the
specials token does not appear anywhere else in this standard, 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.
NO-WS-CTL = %d1-8 / ; US-ASCII control characters
%d11 / ; that do not include the
%d12 / ; carriage return, line feed,
%d14-31 / ; and white space characters
%d127
text = %d1-9 / ; Characters excluding CR and LF
%d11 /
%d12 /
%d14-127 /
obs-text
specials = "(" / ")" / ; Special characters used in
"<" / ">" / ; other parts of the syntax
"[" / "]" /
":" / ";" /
"@" / "\" /
"," / "." /
DQUOTE
No special semantics are attached to these tokens. They are simply
single characters.
3.2.2. 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 = ("\" text) / obs-qp
Where any quoted-pair appears, it is to be interpreted as the text
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
standard where quoted-pair currently appears are ccontent, qcontent,
dcontent, no-fold-quote, and no-fold-literal.
Resnick Standards Track [Page 10]
RFC 2822 Internet Message Format April 2001
3.2.3. 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 3.2.5. Comments may nest.
There are several places in this standard 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 standard, 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) / ; Folding white space
obs-FWS
ctext = NO-WS-CTL / ; Non white space controls
%d33-39 / ; The rest of the US-ASCII
%d42-91 / ; characters not including "(",
%d93-126 ; ")", or "\"
ccontent = ctext / quoted-pair / comment
comment = "(" *([FWS] ccontent) [FWS] ")"
CFWS = *([FWS] comment) (([FWS] comment) / FWS)
Throughout this standard, where FWS (the folding white space token)
appears, it indicates a place where header folding, as discussed in
section 2.2.3, may take place. Wherever header folding appears in a
message (that is, a header field body containing a CRLF followed by
any WSP), header unfolding (removal of the CRLF) is performed before
any further lexical analysis is performed on that header field
according to this standard. 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
Resnick Standards Track [Page 11]
RFC 2822 Internet Message Format April 2001
backslash characters may appear in a comment so long as they appear
as a quoted-pair. Semantically, the enclosing parentheses are not
part of the comment; the comment is what is contained between the two
parentheses. As stated earlier, the "\" in any quoted-pair and the
CRLF in any FWS that appears within the comment are semantically
"invisible" and therefore not part of the comment either.
Runs of FWS, comment or CFWS that occur between lexical tokens in a
structured field header are semantically interpreted as a single
space character.
3.2.4. 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.
atext = ALPHA / DIGIT / ; Any character except controls,
"!" / "#" / ; SP, and specials.
"$" / "%" / ; Used for atoms
"&" / "'" /
"*" / "+" /
"-" / "/" /
"=" / "?" /
"^" / "_" /
"`" / "{" /
"|" / "}" /
"~"
atom = [CFWS] 1*atext [CFWS]
dot-atom = [CFWS] dot-atom-text [CFWS]
dot-atom-text = 1*atext *("." 1*atext)
Both atom and dot-atom are interpreted as a single unit, comprised of
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.
Resnick Standards Track [Page 12]
RFC 2822 Internet Message Format April 2001
3.2.5. Quoted strings
Strings of characters that include characters other than those
allowed in atoms may be represented in a quoted string format, where
the characters are surrounded by quote (DQUOTE, ASCII value 34)
characters.
qtext = NO-WS-CTL / ; Non white space controls
%d33 / ; The rest of the US-ASCII
%d35-91 / ; characters not including "\"
%d93-126 ; or the quote character
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.
3.2.6. 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
Resnick Standards Track [Page 13]
RFC 2822 Internet Message Format April 2001
utext = NO-WS-CTL / ; Non white space controls
%d33-126 / ; The rest of US-ASCII
obs-utext
unstructured = *([FWS] utext) [FWS]
3.3. Date and Time Specification
Date and time 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 may not interpret other occurrences of folding white
space correctly.
date-time = [ day-of-week "," ] date FWS 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
year = 4*DIGIT / obs-year
month = (FWS month-name FWS) / obs-month
month-name = "Jan" / "Feb" / "Mar" / "Apr" /
"May" / "Jun" / "Jul" / "Aug" /
"Sep" / "Oct" / "Nov" / "Dec"
day = ([FWS] 1*2DIGIT) / obs-day
time = time-of-day FWS zone
time-of-day = hour ":" minute [ ":" second ]
hour = 2DIGIT / obs-hour
minute = 2DIGIT / obs-minute
second = 2DIGIT / obs-second
zone = (( "+" / "-" ) 4DIGIT) / obs-zone
Resnick Standards Track [Page 14]
RFC 2822 Internet Message Format April 2001
The day is the numeric day of the month. The year is any numeric
year 1900 or later.
The time-of-day specifies the number of hours, minutes, and
optionally seconds since midnight of the date indicated.
The date and time-of-day SHOULD express local time.
The zone specifies the offset from Coordinated Universal Time (UTC,
formerly referred to as "Greenwich Mean Time") that the date and
time-of-day represent. The "+" or "-" indicates whether the
time-of-day is ahead of (i.e., east of) or behind (i.e., west of)
Universal Time. The first two digits indicate the number of hours
difference from Universal Time, and the last two digits indicate the
number of 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 generated on a system that may be
in a local time zone other than Universal Time and therefore
indicates 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-the-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 [STD12]), and the
zone MUST be within the range -9959 through +9959.
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 ":" [mailbox-list / CFWS] ";"
[CFWS]
Resnick Standards Track [Page 15]
RFC 2822 Internet Message Format April 2001
display-name = phrase
mailbox-list = (mailbox *("," mailbox)) / obs-mbox-list
address-list = (address *("," address)) / obs-addr-list
A mailbox receives mail. It is a conceptual entity which does not
necessarily pertain to file storage. For example, some sites may
choose to print mail on a printer and deliver the output to the
addressee's desk. Normally, a mailbox is comprised of two parts: (1)
an optional display name that indicates the name of the recipient
(which could 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 also 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.
Note: Some legacy implementations used the simple form where the
addr-spec appears without the angle brackets, but included the name
of the recipient in parentheses as a comment following the addr-spec.
Since the meaning of the information in a comment is unspecified,
implementations SHOULD use the full name-addr form of the mailbox,
instead of the legacy form, to specify the display name associated
with a mailbox. Also, because some legacy implementations interpret
the comment, comments generally SHOULD NOT be used in address fields
to avoid confusing such implementations.
When it is desirable to treat several mailboxes as a single unit
(i.e., in a distribution list), the group construct can be used. The
group construct allows the sender to indicate a named group of
recipients. This is done by giving a display name for the group,
followed by a colon, followed by a comma separated list of any number
of mailboxes (including zero and one), and ending with a semicolon.
Because the list of mailboxes can be empty, using the group construct
is also a simple way to communicate to recipients that the message
was sent to one or more named sets of recipients, without actually
providing the individual mailbox address for each 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 "." surrounded by atext
Resnick Standards Track [Page 16]
RFC 2822 Internet Message Format April 2001
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.
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] dcontent) [FWS] "]" [CFWS]
dcontent = dtext / quoted-pair
dtext = NO-WS-CTL / ; Non white space controls
%d33-90 / ; The rest of the US-ASCII
%d94-126 ; characters not including "[",
; "]", or "\"
The domain portion identifies the point to which the mail is
delivered. In the dot-atom form, this is interpreted as an Internet
domain name (either a host name or a mail exchanger name) as
described in [STD3, STD13, STD14]. 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 the mail
transport document [RFC2821]. 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.
Resnick Standards Track [Page 17]
RFC 2822 Internet Message Format April 2001
message = (fields / obs-fields)
[CRLF body]
body = *(*998text CRLF) *998text
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
which are uninterpreted for the purposes of this standard.
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 standard, 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.
fields = *(trace
*(resent-date /
resent-from /
resent-sender /
resent-to /
resent-cc /
resent-bcc /
resent-msg-id))
*(orig-date /
from /
sender /
reply-to /
Resnick Standards Track [Page 18]
RFC 2822 Internet Message Format April 2001
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 a message header 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 number Max number Notes
trace 0 unlimited Block prepended - see
3.6.7
resent-date 0* unlimited* One per block, required
if other resent fields
present - see 3.6.6
resent-from 0 unlimited* One per block - see
3.6.6
resent-sender 0* unlimited* One per block, MUST
occur with multi-address
resent-from - see 3.6.6
resent-to 0 unlimited* One per block - see
3.6.6
resent-cc 0 unlimited* One per block - see
3.6.6
resent-bcc 0 unlimited* One per block - see
3.6.6
resent-msg-id 0 unlimited* One per block - see
3.6.6
orig-date 1 1
from 1 1 See sender and 3.6.2
Resnick Standards Track [Page 19]
RFC 2822 Internet Message Format April 2001
sender 0* 1 MUST occur with multi-
address from - see 3.6.2
reply-to 0 1
to 0 1
cc 0 1
bcc 0 1
message-id 0* 1 SHOULD be present - see
3.6.4
in-reply-to 0* 1 SHOULD occur in some
replies - see 3.6.4
references 0* 1 SHOULD occur in some
replies - see 3.6.4
subject 0 1
comments 0 unlimited
keywords 0 unlimited
optional-field 0 unlimited
The exact interpretation of each field is described in subsequent
sections.
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
Resnick Standards Track [Page 20]
RFC 2822 Internet Message Format April 2001
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
by a single mailbox and the author and transmitter are identical, the
"Sender:" field SHOULD NOT be used. Otherwise, both fields SHOULD
appear.
The originator fields also provide the information required when
replying to a message. When the "Reply-To:" field is present, it
indicates the mailbox(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.
Resnick Standards Track [Page 21]
RFC 2822 Internet Message Format April 2001
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 each of 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.
The "Cc:" field (where the "Cc" means "Carbon Copy" in the sense of
making a copy on a typewriter using carbon paper) contains the
addresses of others who are to receive the message, though the
content of the message may not be directed at them.
The "Bcc:" field (where the "Bcc" means "Blind Carbon Copy") contains
addresses of recipients of the message whose addresses are not to be
revealed to other recipients of the message. There are three ways in
which the "Bcc:" field is used. In the first case, when a message
containing a "Bcc:" field is prepared to be sent, the "Bcc:" line is
removed even though all of the recipients (including those specified
in the "Bcc:" field) are sent a copy of the message. In the second
case, recipients specified in the "To:" and "Cc:" lines each are sent
a copy of the message with the "Bcc:" line removed as above, but the
recipients on the "Bcc:" line get a separate copy of the message
containing a "Bcc:" line. (When there are multiple recipient
addresses in the "Bcc:" field, some implementations actually send a
separate copy of the message to each recipient with a "Bcc:"
containing only the address of that particular recipient.) Finally,
since a "Bcc:" field may contain no addresses, a "Bcc:" field can be
sent without any addresses indicating to the recipients that blind
copies were sent to someone. Which method to use with "Bcc:" fields
is implementation dependent, but refer to the "Security
Considerations" section of this document for a discussion of each.
Resnick Standards Track [Page 22]
RFC 2822 Internet Message Format April 2001
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 SHOULD NOT appear in the "To:" or "Cc:" fields.
Note: Some mail applications have automatic reply commands that
include the destination addresses of the original message in the
destination addresses of the reply. How those reply commands behave
is implementation dependent and is beyond the scope of this document.
In particular, whether or not to include the original destination
addresses when the original message had a "Reply-To:" field is not
addressed here.
3.6.4. Identification fields
Though optional, every message SHOULD have a "Message-ID:" field.
Furthermore, reply messages SHOULD have "In-Reply-To:" and
"References:" fields as appropriate, as described below.
The "Message-ID:" field contains a single unique message identifier.
The "References:" and "In-Reply-To:" field each contain one or more
unique message identifiers, optionally separated by CFWS.
The message identifier (msg-id) is similar in syntax to an angle-addr
construct without the internal CFWS.
message-id = "Message-ID:" msg-id CRLF
in-reply-to = "In-Reply-To:" 1*msg-id CRLF
references = "References:" 1*msg-id CRLF
msg-id = [CFWS] "<" id-left "@" id-right ">" [CFWS]
id-left = dot-atom-text / no-fold-quote / obs-id-left
id-right = dot-atom-text / no-fold-literal / obs-id-right
no-fold-quote = DQUOTE *(qtext / quoted-pair) DQUOTE
Resnick Standards Track [Page 23]
RFC 2822 Internet Message Format April 2001
no-fold-literal = "[" *(dtext / quoted-pair) "]"
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 instantiation 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 which 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.
Resnick Standards Track [Page 24]
RFC 2822 Internet Message Format April 2001
The "References:" field will contain the contents of the parent's
"References:" field (if any) followed by the contents of the parent's
"Message-ID:" field (if any). If the parent message does not contain
a "References:" field but does have an "In-Reply-To:" field
containing a single message identifier, then the "References:" field
will contain the contents of the parent's "In-Reply-To:" field
followed by the contents of the parent's "Message-ID:" field (if
any). If the parent has none of the "References:", "In-Reply-To:",
or "Message-ID:" fields, then the new message will have no
"References:" field.
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 and 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 angle-addr (identical except that 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 "@", 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. Using a date on the left hand side and a domain name or domain
literal on the right hand side makes it possible to guarantee
uniqueness since no two hosts use the same domain name or IP address
at the same time. 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.
Resnick Standards Track [Page 25]
RFC 2822 Internet Message Format April 2001
3.6.5. Informational fields
The informational fields are all optional. The "Keywords:" field
contains a comma-separated list of one or more words or
quoted-strings. The "Subject:" and "Comments:" fields are
unstructured fields as defined in section 2.2.1, and therefore may
contain text or folding white space.
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: " (from the Latin "res", 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
together. Each new set of resent fields is prepended to the message;
that is, the most recent set of resent fields appear earlier in the
message. No other fields in the message are changed when resent
fields are added.
Each of the resent fields corresponds to a particular field elsewhere
in the syntax. For instance, the "Resent-Date:" field corresponds to
the "Date:" field and the "Resent-To:" field corresponds to the "To:"
field. In each case, the syntax for the field body is identical to
the syntax given previously for the corresponding field.
When resent fields are used, the "Resent-From:" and "Resent-Date:"
fields MUST be sent. The "Resent-Message-ID:" field SHOULD be sent.
"Resent-Sender:" SHOULD NOT be used if "Resent-Sender:" would be
identical to "Resent-From:".
Resnick Standards Track [Page 26]
RFC 2822 Internet Message Format April 2001
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. On the other hand, forwarding is also
used to mean when a mail transport program gets a message and
forwards it on to a different destination for final delivery. Resent
header fields are not intended for use with either type of
forwarding.
The resent originator fields indicate the mailbox of the person(s) or
system(s) that resent the message. As with the regular originator
fields, there are two forms: a simple "Resent-From:" form which
contains the mailbox of the individual doing the resending, and the
more complex form, when one individual (identified in the
"Resent-Sender:" field) resends a message on behalf of one or more
others (identified in the "Resent-From:" field).
Note: When replying to a resent message, replies behave just as they
would with any other message, using the original "From:",
Resnick Standards Track [Page 27]
RFC 2822 Internet Message Format April 2001
"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 are a group of header fields consisting of an
optional "Return-Path:" field, and one or more "Received:" fields.
The "Return-Path:" header field contains a pair of angle brackets
that enclose an optional addr-spec. The "Received:" field contains a
(possibly empty) list of name/value pairs followed by a semicolon and
a date-time specification. The first item of the name/value pair is
defined by item-name, and the second item is either an addr-spec, an
atom, a domain, or a msg-id. Further restrictions may be applied to
the syntax of the trace fields by standards that provide for their
use, such as [RFC2821].
trace = [return]
1*received
return = "Return-Path:" path CRLF
path = ([CFWS] "<" ([CFWS] / addr-spec) ">" [CFWS]) /
obs-path
received = "Received:" name-val-list ";" date-time CRLF
name-val-list = [CFWS] [name-val-pair *(CFWS name-val-pair)]
name-val-pair = item-name CFWS item-value
item-name = ALPHA *(["-"] (ALPHA / DIGIT))
item-value = 1*angle-addr / addr-spec /
atom / domain / msg-id
Resnick Standards Track [Page 28]
RFC 2822 Internet Message Format April 2001
A full discussion of the Internet mail use of trace fields is
contained in [RFC2821]. For the purposes of this standard, 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
standard. They MUST conform to the syntax of an optional-field.
This is a field name, made up of the printable US-ASCII characters
except SP and colon, followed by a colon, followed by any text which
conforms to unstructured.
The field names of any optional-field MUST NOT be identical to any
field name specified elsewhere in this standard.
optional-field = field-name ":" unstructured CRLF
field-name = 1*ftext
ftext = %d33-57 / ; Any character except
%d59-126 ; controls, SP, and
; ":".
For the purposes of this standard, any optional field is
uninterpreted.
4. Obsolete Syntax
Earlier versions of this standard 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
interpretation have never been documented. Though some of 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. Taking the
grammar in section 3 and adding the definitions presented in this
section will result in the grammar to use for interpretation of
messages.
Note: This section identifies syntactic forms that any implementation
MUST reasonably interpret. However, there are certainly Internet
messages which 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. To repeat an example, though this document requires
lines in messages to be no longer than 998 characters, silently
Resnick Standards Track [Page 29]
RFC 2822 Internet Message Format April 2001
discarding the 999th and subsequent characters in a line without
warning would still be bad behavior for an 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 s 