Network Working Group                                   P. Resnick, Editor
Internet-Draft                                          QUALCOMM Incorporated
<draft-ietf-drums-msg-fmt-03.txt>                       November 21, 1997

Internet Message Format Standard

0. Status of this memo

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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.

This standard only specifies a syntax 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. These 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", "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 [RFC-2119].

1.2.2 Syntactic notation

This standard uses the Augmented Backus-Naur Form (ABNF) notation specified in
[RFC-2234] 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
[RFC-2234]
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 give the formal syntax and semantics for each of the parts of a
message. 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 which
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 which 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 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". Messages are divided into lines of characters. A
line is
a series of characters which 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".)

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], which
extend this standard to allow for values outside of that range. Discussion of
these mechanisms is not within the scope of this standard.

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 which 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), 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 lexical structure more
restrictive than the unstructured field bodies described above. These are
referred to as "structured" field bodies. Structured field bodies are lines 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 freely
surrounded
by comments (as described in section 3.2.4) as well as space (SP, ASCII value
32) and horizontal tab (HTAB, ASCII value 9) characters, and those surrounding
SP and HTAB 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, 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 SP or HTAB), a CRLF followed by AT
LEAST one SP or HTAB may instead be inserted. 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 SP or
HTAB. Each header field should be treated in its unfolded form for 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] which 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 which 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 tokens, 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 [RFC-2234].

In some of the token definitions, there will be elements whose names start
with
"obs-". These "obs-" elements refer to tokens defined in the obsolete
syntax in
section 4. In all cases, these tokens 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 which must 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 is basically devoted to
defining tokens used in structured header field bodies.

3.2.1 Primitive Tokens

The following are primitive tokens referred to elsewhere in this standard, but
are not otherwise defined in [RFC-2234]. 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 implementors
who use tools which 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 /          ;  which do not include the
                        %d12 /          ;  carriage return, line feed,
                        %d14-31 /       ;  and whitespace 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 attaches 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 should be interpreted as the text character
alone.

3.2.3 Whitespace

The following define the white-space characters used in this standard. See
section 3.2.4 for more information on the use of white-space in the rest of
this standard.

WSP             =       SP / HTAB               ; Whitespace characters
FWS             =       ([*WSP CRLF] 1*WSP) /   ; Folding white-space
                        obs-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) should be 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."

Runs of FWS that occur between lexical tokens are semantically interpreted as
identical to a single space character.

3.2.4 Comments

Strings of characters which are treated as comments may be included in
structured field bodies as characters enclosed in parenthesis. Strings of
characters enclosed in parenthesis are considered comments so long as they do
not appear within a "quoted-string", as defined in section 3.2.6. 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.

ctext           =       NO-WS-CTL /     ; Non-white-space controls

                        %d33-39 /       ; The rest of the US-ASCII
                        %d42-91 /       ;  characters not including "(",
                        %d93-127        ;  ")", or "\"

comment         =       "(" *([FWS] (ctext / quoted-pair / comment)) [FWS] ")"

CFWS            =       *([FWS] comment) (([FWS] comment) / FWS)

A comment is normally used in a structured field body to provide some human
readable informational text. A comment is semantically interpreted as a single
SP. Since a comment is allowed to contain FWS, folding is permitted. 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 token; the
token is what is contained between the two parentheses.

Runs of CFWS are semantically interpreted as a single space character.

3.2.5 Atom

Several tokens in structured header field bodies are simply strings of certain
basic characters. Such tokens are represented as atoms. Two atoms must be
separated by some other token, since putting two atoms next to each other
would
create a single atom.

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 token; the
token
is only the run of atext characters in an atom, or the atext and "."
characters
in a dot-atom.

3.2.6 Quoted strings

Strings of characters which include characters other than those allowed in
atoms may be represented in a quoted string format, where the characters are
surrounded by quote 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

quoted-string   =       [CFWS]
                        DQUOTE *([FWS] (qtext / quoted-pair)) [FWS] DQUOTE
                        [CFWS]

A quoted-string is treated as a single symbol. 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 token; the token is
what is contained between the two quote characters.

3.2.7 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 comments
and folding whitespace are permitted throughout the date-time
specification, it
is recommended that only a single space be used where CFWS is required, and
that comments be limited to the end of the date-time specification; some older
implementations may not interpret other occurrences of comments and folding
whitespace correctly.

date-time       =       [ day-of-week "," ] date CFWS time [CFWS]

day-of-week     =       CFWS day-name CFWS

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

date            =       day month year

year            =       ([CFWS] 4*DIGIT [CFWS]) / obs-year

month           =       CFWS month-name CFWS

month-name      =       "Jan" / "Feb" / "Mar" / "Apr" /
                        "May" / "Jun" / "Jul" / "Aug" /
                        "Sep" / "Oct" / "Nov" / "Dec"

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

time            =       time-of-day CFWS zone

time-of-day     =       hour ":" minute [ ":" second ]

hour            =       [CFWS] 2DIGIT [CFWS]

minute          =       [CFWS] 2DIGIT [CFWS]

second          =       [CFWS] 2DIGIT [CFWS]

zone            =       (( "+" / "-" ) 4DIGIT) / obs-zone

The day is the numeric day of the month. The year is any numeric year in the
common era.

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

display-name    =       phrase

group-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 which 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
confusion.

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
group name, 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 both 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 whitespace 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] (dtext / quoted-pair)) [FWS] "]"
                        [CFWS]

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 [DNS]. 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. In a
message identifier (described in section 3.6.4), it is an identifying string
that is unique to a message generated on a particular host. It is otherwise
uninterpreted in this standard.

3.5 Overall message syntax

A message consists of header fields, optionally followed by a message body. In
a message body, though all of the characters listed in the text rule MAY be
used, the US-ASCII control characters(values 1 through 8, 11, 12, and 14
through 31) SHOULD NOT be used. Also, though the lines in the body MAY be a
maximum of 998 characters excluding the CRLF, lines SHOULD be limited to 78
characters excluding the CRLF.

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               infinite        Block prepended - see 3.6.7

resent-date     0*              infinite*       One per block, required if
                                                other resent fields present
                                                - see 3.6.6

resent-from     0               infinite*       One per block - see 3.6.6

resent-sender   0*              infinite*       One per block, MUST occur
                                                with multi-address
                                                resent-from - see 3.6.6

resent-to       0               infinite*       One per block - see 3.6.6

resent-cc       0*              infinite*       One per block, SHOULD only
                                                occur with resent-to - see
                                                3.6.6

resent-bcc      0               infinite*       One per block - see 3.6.6

resent-id       0               infinite*       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*              SHOULD occur only with to -
                                                see 3.6.3
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               1

keywords        0               1

optional-field  0               infinite

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 in 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 should 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 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 mailboxes.

from            =       "From:" mailbox-list CRLF

sender          =       "Sender:" mailbox CRLF

reply-to        =       "Reply-To:" mailbox-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 go in
the
"Sender:" field and the mailbox of the actual author would go 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 to reply 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 which 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). Both the "To:" field and the "Bcc:" field MAY occur alone, but the
"Cc:" field SHOULD only be present if the "To:" field is also present.

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 receives a
copy
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
should 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 should not 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:" and "In-Reply-To:" field each contain a single unique
message
identifier. The "References:" field contains one or more unique message
identifiers, optionally separated by CFWS.

The message identifier is simply the same syntax as an addr-spec construct
enclosed in the angle bracket characters, "<" and ">".

message-id      =       "Message-ID:" identifier CRLF

in-reply-to     =       "In-Reply-To:" identifier CRLF

references      =       "References:" identifier *([CFWS] identifier) CRLF

identifier      =       "<" id-left-side "@" id-right-side ">"

id-left-side    =       dot-atom-text / no-fold-quote / obs-id-left-side

id-right-side   =       dot-atom-text / no-fold-literal / obs-id-right-side

no-fold-quote   =       DQUOTE *(qtext / quoted-pair) DQUOTE

The "Message-ID:" field provides a unique identifier which refers to a
particular version of a particular message. The uniqueness of the message
identifier is guaranteed by the host which generates it (see below). This
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 should each receive
new
message identifiers.

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 should be 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 should be copied to the "References:" field in the new
message, appending to it the contents of the old "In-Reply-To:" field (if its
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 message identifier itself is a domain-dependent unique identifier. The
domain portion of the identifier SHOULD be the domain name of the host on
which
it was created, to guarantee uniqueness. The local-part portion of the
identifier MAY be any dot-atom or quoted-string. However, the entire
identifier
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 identifier MUST guarantee that the identifier is
unique. There are several algorithms that can be used to accomplish this.
Since
the identifier has an similar syntax to addr-spec (identical except that
comments and folding whitespace are not allowed), a good method is to put the
domain name or a domain literal IP address of the host on which the identifier
was created on the right hand side of the "@", and on the left hand side,
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). 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 should be using 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. 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 which 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 should be prepended to the message; that is, the most recent set of
resent fields should appear earlier in the message. No other fields in the
message should be 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-Cc:" field SHOULD NOT be sent if the "Resent-To:"
field is
not present. 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:" identifier 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
actions
on messages.

Note: Reintroducing a message into the transport system and using resent
fields
is a different operation from "forwarding". Forwarding a message is to make it
the body of a new message. A forwarded message does not appear to have come
from the original sender, but is an entirely new message from the forwarder of
the message. Resent headers are not intended for use with 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 should 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 when forming 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 which enclose an optional addr-
spec. The "Received:" field contains a series of token-value pairs followed by
a semicolon and a date-time specification. The first item of the token value
pair is defined by token-name and the second item is either an atom or a
quoted-string. Further restrictions may be applied to the syntax of the trace
fields by standards which provide for their use, such as [SMTP].

trace           =       [return]
                        1*received

return          =       "Return-Path:" path CRLF

path            =       [CFWS] "<" ([CFWS] / addr-spec) ">" [CFWS]

received        =       "Received:" [CFWS] *token-value ";" date-time CRLF

token-value     =       [CFWS] token-name CFWS word

token-name      =       ALPHA *(["-"] (ALPHA / DIGIT))

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 basically a
field
name, made up of the printable US-ASCII characters except SP and colon,
followed by a colon, followed by unstructured text.

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

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.

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 3.2.4 regarding comments and folding whitespace does not
apply.
See the discussion of folding whitespace in section 4.2 below.

Finally, certain characters which 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.

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

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

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

obs-phrase      =       word *(word / "." / CFWS)

4.2 Obsolete folding whitespace

In the obsolete syntax, any amount of folding whitespace 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 whitespace.

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 which were
used in
earlier versions of this standard.

obs-year        =       [CFWS] 2*DIGIT [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" through
                        %d97-105 /              ; "Z", both upper and lower
                        %d107-122               ; case

Where a two or three digit year occurs in a date, the year should be
interpreted as follows: If a two digit year is encountered whose value is
between 00 and 49, the year should be 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
should be 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), the 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".

Other multi-character (usually between 3 and 5) alphabetic time zones have
been
used in Internet messages. Any of these time zones SHOULD be considered
equivalent too "-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 whitespace 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 identifier allow interspersed
CFWS, making them syntactically identical to local-part and domain
respectively.

obs-message-id  =       "Message-ID" *WSP ":" identifier CRLF

obs-in-reply-to =       "In-Reply-To" *WSP ":" *(phrase / identifier) CRLF

obs-references  =       "References" *WSP ":" *(phrase / identifier) CRLF

obs-id-left     =       local-part

obs-id-right    =       domain

For purposes of interpretation, the phrases in the "In-Reply-To:" and
"References:" fields may be 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 whitespace, 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 ":" identifier CRLF

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

As with other resent fields, the "Resent-Reply-To:" field should 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 ":" *([CFWS] text) CRLF

obs-received    =       "Received" *WSP ":" *([CFWS] text) CRLF

4.5.8 Obsolete optional fields

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

5. Security Considerations

Care should 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
revealing that the message went to the blind recipient. When the second method
from section 3.6.3 is used, the blind recipients 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 must still 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

[RFC-822]

[RFC-2045]

[RFC-2046]

[RFC-2047]

[RFC-2048]

[RFC-2049]

[SMTP]

[RFC-2119]

[RFC-2234]

[ASCII]

[STD-12]

[DNS]

[ISO-2022]

7. Author's Address

Peter W. Resnick
QUALCOMM Incorporated
6455 Lusk Boulevard
San Diego, CA 92121-2779
Phone: +1 619 651 4478
FAX: +1 619 651 5334
e-mail: presnick@qualcomm.com

Grammar and syntax comments are welcome. 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 Adressing examples

The following are examples of messages which 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.tld>
To: Mary Smith <mary@harry.nil>
Subject: Saying Hello
Date: Fri, 21 Nov 1997 09:55:06 -0600
Message-ID: <1234@local.machine.tld>

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@hiccup.tld>
To: Mary Smith <mary@harry.nil>, jdoe@machine.tld, Who? <one@here.nil>
Cc: <boss@test.nil>, "System Service's Box" <sysservices@hiccup.tld>
Date: Tue, 1 Jul 2003 10:52:37 +0200
Message-ID: <5678.21-Nov-1997@hiccup.tld>

Hi everyone.
----

Note that the display names for Joe Q. Public and System Service's Box needed
to be enclosed in double-quotes because the contain the period and
single-quote
characters, while the display name for Who? could appear without them because
the question mark is legal in an atom. Notice also that jdoe@machine.tld and
boss@test.nil have no display names associated with them at all, and
joe@machine.tld uses the simpler address form without the angle brackets.

A.1.3 Group addresses

----
From: Pete <pete@silly.nil>
To: A Group:Chris Jones <c@public.tld>,joe@where.nil,John <jdoe@one.nil>;
Cc: Undisclosed recipients:;
Date: Sat, 15 May 1869 23:32:54 -0330
Message-ID: <testabcd.1234@silly.nil>

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 Whitespace and comments

A.3 Reply messages

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

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

----
From: Mary Smith <mary@harry.nil>
To: John Doe <jdoe@machine.tld>
Subject: Re: Saying Hello
Date: Fri, 21 Nov 1997 10:01:10 -0600
Message-ID: <3456@harry.nil>
In-Reply-To: <1234@local.machine.tld>
References: <1234@local.machine.tld>

This is a reply to your hello.
----

----
To: Mary Smith <mary@harry.nil>
From: John Doe <jdoe@machine.tld>
Subject: Re: Saying Hello
Date: Fri, 21 Nov 1997 11:00:00 -0600
Message-ID: <abcd.1234@local.machine.tld>
In-Reply-To: <3456@harry.nil>
References: <1234@local.machine.tld> <3456@harry.nil>

This is a reply to your reply.
----

A.4 Resent messages

A.5 Messages with trace fields

A.6 Other obsoleted forms

Appendix B - Differences from earlier standards

[Editor's Note: This will be real eventually, for now just changes in this
draft.

1. Fix FWS, CFWS, comment, obs-text, dot-atom, and atom.
2. Change "unknown time zone" wording
3. NUL character, not NULL
4. Got rid of CHAR
5. should -> SHOULD in 4.5.3
6. null elements in domain-list
7. Added obs-mailbox-list and obs-address-list for null elements
8. null elements in obs-keywords
9. Change [CFWS] to *WSP before ":" in obs-fields
10. Change name-domain to addr-spec
11. Typos
12. Unstructured FWS is optional.
13. Re:
14. identifier -> message-id in 3.6
15. Resent-Message-ID - address-list -> identifier
16. FWS optional in quoted-string, domain-literal, atom, and comment
17. Identifier redefined
18. Message-ID text added
19. Clarifications in folding
20. Changed title (*shrug*)
21. Clarified References and In-Reply-To formation
22. Added "group-name" and "display-name" in addressing
23. Added some examples
24. Bcc may be empty.
25. Added some Reply-To text in lieu of something new

To do list:

Specifically talk about X-* headers.
Change reply-to yet again?
Bibliography
Acknowledgements
Examples
Differences

]