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 
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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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