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Versions: 00 01 02 03 04 05 06 07 08 RFC 2822

Network Working Group                           P. Resnick, Editor
Internet-Draft                                  QUALCOMM Incorporated
<draft-ietf-drums-msg-fmt-07.txt>               January 8, 1999

Internet Message Format Standard

Status of this memo

This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas, and
its working groups. Note that other groups may also distribute working
documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference material
or to cite them other than as "work in progress."

To learn the current status of any Internet-Draft, please check the
"1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
munnari.oz.au (Pacific Rim), ftp.ietf.org (US East Coast), or
ftp.isi.edu (US West Coast).

Note: Though this document uses the word "standard" in both the title
and the body of the text, it is of course still an Internet Draft and is
NOT actually a standard until it has been approved and published as an
RFC.

Copyright Notice

Copyright (C) The Internet Society (1998).  All Rights Reserved.

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 822,
"Standard for the Format of ARPA Internet Text Messages" [RFC-822],
updating it to reflect current practice and incorporating incremental
changes that were specified in other RFCs [STD-3].

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 [RFC-2045, RFC-
2046, RFC-2049], 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 [SMTP] 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.

Some message systems may store messages in formats that differ from the
one specified in this standard. This specification is intended strictly
as a definition of what message content format is to be passed between
hosts.

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 [BCP-14].

1.2.2. Syntactic notation

This standard uses the Augmented Backus-Naur Form (ABNF) notation
specified in [ABNF] 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 [ABNF] 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 will lay 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 will specify formal ABNF rules for the structure of each part
of a message (syntax) and describe the relationship between those parts
and their meaning in the context of a message (the semantics). That is,
it will describe 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
will include 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.

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 author's address and instructions on where to
send comments.

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.

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".

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 [RFC-2045, RFC-2046, RFC-2047,
RFC-2048, RFC-2049], 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".) Each line of characters MUST be limited to 998 characters,
and SHOULD be limited to 78 characters, excluding the CRLF.

Note: 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.
The 78 character recommendation is due to limitations in many
implementations that display these messages which may truncate the
display of more than 78 characters per line. Of course, even though
these limitations are put on messages, interpreters of messages would do
well to handle an arbitrarily large number of characters in a line,
including for display, for robustness' sake.

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).

2.2. Header Fields

Header fields are lines that have a specific syntax. Header fields are
all 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 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, 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 [RFC-2045, RFC-2046, RFC-2048, RFC-2049]
that extend this standard 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 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 [ABNF].

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 [ABNF]. Some of them will not
appear anywhere else in the syntax, but they are convenient to refer to
in other parts of this document.

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
                        %d11-12 /
                        %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.

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
parenthesis. The following defines the folding white space (FWS) and
comment contructs.

Strings of characters enclosed in parenthesis 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 "(",
                        %d93-127        ;  ")", 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 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.

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 "\"
                        %d93-127        ;  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

unstructured    =       *([FWS] text)

3.3. Date and Time Specification

Date and time occur in several header fields of a message. 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 only a single space be used where FWS is required
and no space be used where FWS is optional in the date-time
specification; 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 [FWS]) / obs-day-of-week

day-name        =       "Mon" / "Tue" / "Wed" / "Thu" /
                        "Fri" / "Sat" / "Sun"

date            =       day month year

year            =       ([FWS] 4*DIGIT [FWS]) / 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 [FWS]) / 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

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 or behind 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.

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 [STD-12]), 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 / obs-mailbox

name-addr       =       [display-name] [CFWS] "<" addr-spec ">" [CFWS]

group           =       display-name ":" [mailbox-list / CFWS] ";"
                        [CFWS]

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 if a name of the
recipient is being used instead of the legacy form. Also, because some
legacy implementations interpret the comment, comments SHOULD NOT
generally 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 indicate in the message that a set of recipients was sent the
message without actually providing the individual mailbox address for
each of the 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 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
                        %d94-127        ;  characters not including "[",
                                        ;  "]", or "\"

The domain portion is a fully qualified identifier for an Internet host.
For example, in a mailbox address, it is the host on which the
particular mailbox resides. 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 [STD-3, STD-13, STD-14]. 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 [SMTP]. 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 MAY be a maximum of 998 characters excluding
the CRLF, but it is RECOMMENDED that lines be limited to 78 characters
excluding the CRLF. (See the note in section 2.1 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.

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-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 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-id       0               unlimited*      One per block - see
                                                3.6.6

orig-date       1               1

from            1               1               See sender and 3.6.2

sender          0*              1               MUST occur with multi-
                                                address from - see 3.6.2

reply-to        0               1

to              0               1

cc              0               1

bcc             0               1

message-id      0*              1               SHOULD be present - see
                                                3.6.4

in-reply-to     0*              1               SHOULD occur in some
                                                replies - see 3.6.4

references      0*              1               SHOULD occur in some
                                                replies - see 3.6.4

subject         0               1

comments        0               unlimited

keywords        0               unlimited

optional-field  0               unlimited

The exact interpretation of each field is described in subsequent
sections.

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 laptop 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 by a single mailbox and the
author and transmitter are identical, the "From:" field SHOULD be used
and 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 be sent to the mailbox(es) specified in the "From:" field.

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 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.

When a message is a reply to another message, the mailboxes of the
authors of the original message (the mailboxes in the "From:" or "Reply-
To:" fields) MAY appear in the "To:" field of the reply, since that
would normally be the primary recipient. 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 addr-spec
construct enclosed in the angle bracket characters, "<" and ">", 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

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). If the
original message contains a "Message-ID:" field, the contents of that
field body are copied into the body of an "In-Reply-To:" field and into
the body of a "References:" field in the new message. If the original
message contains a "References:" field and/or an "In-Reply-To:" field
already (hence a reply to a reply), the contents of the old
"References:" field are copied to the "References:" field in the new
message, appending to it the contents of the old "In-Reply-To:" field
(if its message identifier was not already in the "References:" field)
and the contents of the "Message-ID:" field of the original message. In
this way, a "thread" of conversation can be established.

The msg-id itself is a domain-dependent unique identifier. The domain
portion of the msg-id SHOULD be the domain name of the host on which it
was created, to guarantee uniqueness. The local-part portion of the msg-
id MAY be any dot-atom or quoted-string. However, the entire msg-id MUST
be globally unique. In order to do this, a common practice is to form
the local-part by using a combination of the current absolute time and
some other currently unique identifier on the host (for example a system
process identifier).

The message identifier 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 an similar syntax to addr-spec
(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.

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 only intended to have 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 if this occurs multiple times. 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:".

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: First, there is a sense of forwarding in that a mail reading
program can be told by the user to forward a copy of the message to
another person is to make a message the body of a 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. Second, 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:", "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 [SMTP].

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      =       addr-spec / atom / domain / msg-id

A full discussion of the Internet mail use of trace fields is contained
in [SMTP]. 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, the meaning of 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 that 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 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 malformed data to be irrevocably lost by any implementation. To
repeat an example, though this document requires lines in messages to be
no longer than 998 characters, silently 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 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 3.2.3 regarding 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. 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. The obs-char and obs-qp elements each add ASCII value
0. Bare CR and bare LF are added to obs-text. The period character is
added to obs-phrase.

Note: The period character in obs-phrase is not a form that was allowed
in earlier versions of this or any other standard. 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. In the future, period may appear in the regular
syntax of phrase.

obs-qp          =       "\" (%d0-127)

obs-text        =       *LF *CR *(obs-char *LF *CR)

obs-char        =       %d0-9 / %d11 /          ; %d0-127 except CR and
                        %d12 / %d14-127         ;  LF

obs-phrase      =       word *(word / "." / 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 case, bare CR and bare LF are used
simply as 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*WSP *(CRLF 1*WSP)

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 specifications
that were used in earlier versions of this standard. It also permits
comments and folding white space between many of the tokens.

obs-day-of-week =       [CFWS] day-name [CFWS]

obs-year        =       [CFWS] 2*DIGIT [CFWS]

obs-month       =       CFWS month-name CFWS

obs-day         =       [CFWS] 1*2DIGIT [CFWS]

obs-hour        =       [CFWS] 2DIGIT [CFWS]

obs-minute      =       [CFWS] 2DIGIT [CFWS]

obs-second      =       [CFWS] 2DIGIT [CFWS]

obs-zone        =       "UT" / "GMT" /          ; Universal Time
                                                ; North American UT
                                                ; offsets
                        "EST" / "EDT" /         ; Eastern:  - 5/ - 4
                        "CST" / "CDT" /         ; Central:  - 6/ - 5
                        "MST" / "MDT" /         ; Mountain: - 7/ - 6
                        "PST" / "PDT" /         ; Pacific:  - 8/ - 7

                        %d65-73 /               ; Military zones - "A"
                        %d75-90 /               ; through "I" and "K"
                        %d97-105 /              ; through "Z", both
                        %d107-122               ; upper and lower case

Where a two or three digit year occurs in a date, the year is to be
interpreted as follows: If a two digit year is encountered whose value
is between 00 and 49, the year is interpreted by adding 2000, ending up
with a value between 2000 and 2049. If a two digit year is encountered
with a value between 50 and 99, or any three digit year is encountered,
the year is interpreted by adding 1900.

In the obsolete time zone, "UT" and "GMT" are indications of "Universal
Time" and "Greenwich Mean Time" respectively and are both semantically
identical to "+0000". The remaining three character zones are the US
time zones. The "T" is simply "Time" and the "E", "C", "M", and "P" are
"Eastern", "Central", "Mountain" and "Pacific". When followed by "S"
(for "Standard"), each of these are equivalent to "-0500", "-0600", "-
0700", and "-0800" respectively. When followed by "D" (for "Daylight" or
summer time), they each add an hour and are therefore "-0400", "-0500",
"-0600", and "-0700" respectively. The 1 character military time zones
were defined in a non-standard way in [RFC-822] 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 [RFC-822], 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".

4.4. Obsolete Addressing

There are three 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). Finally, 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.

obs-mailbox     =       addr-spec / [display-name] obs-route-addr

obs-route-addr  =       [CFWS] "<" [obs-route] addr-spec ">" [CFWS]

obs-route       =       [CFWS] obs-domain-list ":" [CFWS]

obs-domain-list =       "@" domain *(*(CFWS / "," ) [CFWS] "@" domain)

obs-local-part  =       atom *("." atom)

obs-domain      =       atom *("." atom)

obs-mbox-list   =       *([mailbox] [CFWS] "," [CFWS])

obs-addr-list   =       *([address] [CFWS] "," [CFWS])

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.

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-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 which 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 ":" mailbox-list CRLF

4.5.3. Obsolete destination address fields

obs-to          =       "To" *WSP ":" address-list CRLF

obs-cc          =       "Cc" *WSP ":" address-list CRLF

obs-bcc         =       "Bcc" *WSP ":" (address-list / [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.

4.5.5. Obsolete informational fields

obs-subject     =       "Subject" *WSP ":" unstructured CRLF

obs-comments    =       "Comments" *WSP ":" unstructured CRLF

obs-keywords    =       "Keywords" *WSP ":" *([phrase] ",") CRLF

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]) CRLF

obs-resent-mid  =       "Resent-Message-ID" *WSP ":" msg-id CRLF

obs-resent-rply =       "Resent-Reply-To" *WSP ":" address-list CRLF

As with other resent fields, the "Resent-Reply-To:" field is to be
treated as trace information only.

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 [SMTP].

obs-return      =       "Return-Path" *WSP ":" path CRLF

obs-received    =       "Received" *WSP ":" name-val-list CRLF

obs-path        =       obs-route-addr

4.5.8. Obsolete optional fields

obs-optional    =       field-name *WSP ":" unstructured CRLF

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 ASCII control characters which remap the keyboard
or permit other modifications to the terminal which could lead to denial
of service or even damaged data. 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. [RFC-2045, RFC-2046, RFC-2047, RFC-2048, RFC-2049],
[ISO-2022]) and therefore should not be stripped indiscriminately.

Transmission of non-text objects in messages raises additional security
issues. These issues are discussed is [RFC-2045, RFC-2046, RFC-2047,
RFC-2048, RFC-2049].

Many implementations use the "Bcc:" (blind carbon copy) field described
in section 3.6.3 to facilitate sending messages to recipients without
revealing the addresses of one or more of the addressees to the other
recipients. Mishandling this use of "Bcc:" has implications for
confidential information that might be revealed, which could eventually
lead to security problems through knowledge of even the existence of a
particular mail address. For example, if using the first method
described in section 3.6.3, where the "Bcc:" line is removed from the
message, blind recipients have no explicit indication that they have
been sent a blind copy, except insofar as their address does not appear
in the message header. Because of this, one of the blind addressees
could potentially send a reply to all of the shown recipients and
accidentally reveal that the message went to the blind recipient. When
the second method from section 3.6.3 is used, the blind recipient's
address appears in the "Bcc:" field of a separate copy of the message.
If the "Bcc:" field sent contains all of the blind addressees, all of
the "Bcc:" recipients will be seen by each "Bcc:" recipient. Even if a
separate message is sent to each "Bcc:" recipient with only the
individual's address, implementations still need to be careful to
process replies to the message as per section 3.6.3 so as not to
accidentally reveal the blind recipient to other recipients.

6. Bibliography

[ASCII] American National Standards Institute (ANSI), Coded Character
Set - 7-Bit American National Standard Code for Information Interchange,
ANSI X3.4, 1986.

[BCP-14] Bradner, S., Key words for use in RFCs to Indicate Requirement
Levels, BCP 14, RFC 2119, March 1997.

[ISO-2022] International Organization for Standardization (ISO),
Information processing - ISO 7-bit and 8-bit coded character sets - Code
extension techniques, Third edition - 1986-05-01, ISO 2022, 1986.

[RFC-822] Crocker, D., Standard for the Format of ARPA Internet Text
Messages. STD 11, RFC 822. August 1982.

[RFC-2045] Freed, N. and Borenstein, N.,  Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies, RFC 2045,
November 1996.

[RFC-2046] Freed, N. and Borenstein, N., Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types, RFC 2046, November 1996.

[RFC-2047] Moore, K., Multipurpose Internet Mail Extensions (MIME) Part
Three: Message Header Extensions for Non-ASCII Text, RFC 2047, November
1996.

[RFC-2048] Freed, N., Klensin, J., and Postel, J., Multipurpose Internet
Mail Extensions (MIME) Part Four: Format of Internet Message Bodies, RFC
2048, November 1996.

[RFC-2049] Freed, N. and Borenstein, N., Multipurpose Internet Mail
Extensions (MIME) Part Five: Conformance Criteria and Examples, RFC
2049, November 1996.

[ABNF] Crocker, D., Editor, and Overell, P., Augmented BNF for Syntax
Specifications: ABNF, Work In Progress.

[SMTP] Klensin, J., Editor, Simple Mail Transfer Protocol, Work In
Progress.

[STD-3] Braden, R., Host Requirements, STD 3, RFC 1122 and RFC 1123,
October 1989.

[STD-12] Mills, D., Network Time Protocol, STD 12, RFC 1119, September
1989.

[STD-13] Mockapetris, P., Domain Name System, STD 13, RFC 1034 and RFC
1035 November 1987.

[STD-14] Partridge, C., Mail Routing and the Domain System, STD 14, RFC
974, January 1986.

7. Editor's Address

Peter W. Resnick
QUALCOMM Incorporated
6455 Lusk Boulevard
San Diego, CA 92121-2779
USA
Phone: +1 619 651 4478
FAX: +1 619 651 1102
e-mail: presnick@qualcomm.com

[Editor's note: Grammar and syntax comments are welcome sent directly to
the editor. Substantive comments on this document should be directed to
the DRUMS working group. The subscription address is
<drums-request@cs.utk.edu>.]

8. Acknowledgements

[TBD]

Appendix A. Examples messages

This section presents a selection of messages. These are intended to
assist in the implementation of this standard, 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.

Messages are delimited in this section 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, 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".
----

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>, "Gaint; \"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
construct). 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:Chris Jones <c@a.test>,joe@where.test,John <jdoe@one.test>;
Cc: Undisclosed recipients:;
Date: Sat, 13 Feb 1869 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 firsts sends a message to Mary, Mary
then replies to John's message, and then John replies to Mary's reply
message.

Note especially the "Message-ID:", "References:", and "In-Reply-To:"
fields in each message.

----
From: John Doe <jdoe@machine.example>
To: Mary Smith <mary@example.net>
Subject: Saying Hello
Date: Fri, 21 Nov 1997 09:55:06 -0600
Message-ID: <1234@local.machine.example>

This is a message just to say hello.
So, "Hello".
----

When sending replies, the Subject field is often retained, though
prepended with "Re: " as described in 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.tld>
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:

----
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.

A.4. Messages with trace fields

As messages are sent through the transport system as described in
[SMTP], trace fields are prepended to the message. The following is an
example of what those trace fields might look like. Note that there is
some folding white space in the first one since these lines can be long.

----
Received: from x.y.test
   by example.net
   via TCP
   with ESMTP
   id ABC12345
   for <mary@example.net>;  21 Nov 1997 10:05:43 -0600
Received: from machine.example by x.y.test; 21 Nov 1997 10:01:22 -0600
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".
----

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 wonderful \) chap) <pete(his account)@silly.test(his host)>
To:A Group(Some people)
     :Chris Jones <c@(Chris's host.)public.example>,
         joe@example.org,
  John <jdoe@one.test> (my dear friend); (the end of the group)
Cc:(Empty list)(start)Undisclosed recipients  :(nobody(that I know))  ;
Date: Sat,
      13
        Feb
          1869
      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 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 below example 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 <@machine.tld: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.

----
>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. 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 standards

This appendix contains a list of changes that have been made in the
Internet Message Format from earlier standards, specifically [RFC-822]
and [STD-3]. Items marked with an asterisk (*) below are items which
appear in section 4 of this document and therefore can no longer be
generated.

1. Period allowed in obsolete form of phrase.
2. ABNF moved out of document to [ABNF].
3. Four or more digits allowed for year.
4. Header field ordering (and lack thereof) made explicit.
5. Encrypted header field removed.
6. Received syntax loosened to allow any token/value pair.
7. Specifically allow and give meaning to "-0000" time zone.
8. Folding white space is not allowed between every token.
9. Requirement for destinations removed.
10. Forwarding and resending redefined.
11. Extension header fields no longer specifically called out.
12. ASCII 0 (null) removed.*
13. Folding continuation lines cannot contain only white space.*
14. Free insertion of comments not allowed in date.*
15. Non-numeric time zones not allowed.*
16. Two digit years not allowed.*
17. Three digit years interpreted, but not allowed for generation.
18. Routes in addresses not allowed.*
19. CFWS within local-parts and domains not allowed.*
20. Empty members of address lists not allowed.*
21. Folding white space between field name and colon not allowed.*
22. Comments between field name and colon not allowed.
23. Tightened syntax of in-reply-to and references.*
24. CFWS within msg-id not allowed.*
25. Tightened semantics of resent fields as informational only.
26. Resent-Reply-To not allowed.*
27. No multiple occurrences of fields (except resent and received).*
28. Free CR and LF not allowed.*
29. Routes in return path not allowed.*
30. Line length limits specified.
31. Bcc more clearly specified.

Full Copyright Statement

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