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RFC #  822
Obsoletes:  RFC #733  (NIC #41952)

RFC822: Standard for ARPA Internet Text Messages

Revised by David H. Crocker

Dept. of Electrical Engineering

University of Delaware, Newark, DE 19711

Network: DCrocker @ UDel-Relay

Partial Hypertext conversion by Tim Berners-Lee/CERN

TABLE OF CONTENTS

PREFACE ....................................................   ii1. INTRODUCTION ...........................................    11.1.  Scope ............................................    11.2.  Communication Framework ..........................    22. NOTATIONAL CONVENTIONS .................................    33. LEXICAL ANALYSIS OF MESSAGES 3.1. General Description 3.2. Header Field Definitions 3.3. Lexical Tokens 3.4. Clarifications 4. MESSAGE SPECIFICATION ..................................   174.1.  Syntax ...........................................   174.2.  Forwarding .......................................   194.3.  Trace Fields .....................................   204.4.  Originator Fields ................................   214.5.  Receiver Fields ..................................   234.6. Reference Fields .................................   234.7. Other Fields .....................................   245. DATE AND TIME SPECIFICATION ............................   265.1.  Syntax ...........................................   265.2.  Semantics ........................................   266. ADDRESS SPECIFICATION ..................................   276.1.  Syntax ...........................................   276.2.  Semantics ........................................   276.3.  Reserved Address .................................   337. BIBLIOGRAPHY ...........................................   34APPENDIXA. EXAMPLES ...............................................   36
B. SIMPLE FIELD PARSING ...................................   40
C. DIFFERENCES FROM RFC #733 ..............................   41
D. ALPHABETICAL LISTING OF SYNTAX RULES ...................   44

PREFACE

By 1977, the Arpanet employed several informal standards for the text messages (mail) sent among its host computers. It was felt necessary to codify these practices and provide for those features that seemed imminent. The result of that effort was Request for Comments (RFC) #733, "Standard for the Format of ARPA Network Text Message", by Crocker, Vittal, Pogran, and Henderson. The specification attempted to avoid major changes in existing software, while permitting several new features.

This document revises the specifications in RFC #733, in order to serve the needs of the larger and more complex ARPA Internet. Some of RFC #733's features failed to gain adequate acceptance. In order to simplify the standard and the software that follows it, these features have been removed. A different addressing scheme is used, to handle the case of inter-network mail; and the concept of re-transmission has been introduced.

This specification is intended for use in the ARPA Internet. However, an attempt has been made to free it of any dependence on that environment, so that it can be applied to other network text message systems.

The specification of RFC #733 took place over the course of one year, using the ARPANET mail environment, itself, to provide an on-going forum for discussing the capabilities to be included. More than twenty individuals, from across the country, participated in the original discussion. The development of this revised specification has, similarly, utilized network mail-based group discussion. Both specification efforts greatly benefited from the comments and ideas of the participants.

The syntax of the standard, in RFC #733, was originally specified in the Backus-Naur Form (BNF) meta-language. Ken L. Harrenstien, of SRI International, was responsible for recoding the BNF into an augmented BNF that makes the representation smaller and easier to understand.

1. INTRODUCTION

1.1. SCOPE

This standard specifies a syntax for text messages that  are
sent  among  computer  users, within the framework of "electronic
mail".  The standard supersedes  the  one  specified  in  ARPANET
Request  for Comments #733, "Standard for the Format of ARPA Net-
work Text Messages".In this context, messages are viewed as having  an  envelope
and  contents.   The  envelope  contains  whatever information is
needed to accomplish transmission  and  delivery.   The  contents
compose  the object to be delivered to the recipient.  This stan-
dard applies only to the format and some of the semantics of mes-
sage  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 stan-
dard 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 specifica-
tion 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 for-mats  used  by sites, the specific message system featuresthat they are expected to support, or any of  the  charac-teristics  of  user interface programs that create or readmessages.A distinction should be made between what the  specification
REQUIRES  and  what  it ALLOWS.  Messages can be made complex and
rich with formally-structured components of information or can be
kept small and simple, with a minimum of such information.  Also,
the standard simplifies the interpretation  of  differing  visual
formats  in  messages;  only  the  visual  aspect of a message is
affected and not the interpretation  of  information  within  it.
Implementors may choose to retain such visual distinctions.The formal definition is divided into four levels.  The bot-
tom level describes the meta-notation used in this document.  The
second level describes basic lexical analyzers that  feed  tokens
to  higher-level  parsers.   Next is an overall specification for
messages; it permits distinguishing individual fields.   Finally,
there is definition of the contents of several structured fields.

1.2. COMMUNICATION FRAMEWORK

Messages consist of lines of text.   No  special  provisions
are  made for encoding drawings, facsimile, speech, or structured
text.  No significant consideration has been given  to  questions
of  data  compression  or to transmission and storage efficiency,
and the standard tends to be free with the number  of  bits  con-
sumed.   For  example,  field  names  are specified as free text,
rather than special terse codes.A general "memo" framework is used.  That is, a message con-
sists of some information in a rigid format, followed by the main
part of the message, with a format that is not specified in  this
document.   The  syntax of several fields of the rigidly-formated
("headers") section is defined in  this  specification;  some  of
these fields must be included in all messages.The syntax  that  distinguishes  between  header  fields  is
specified  separately  from  the  internal  syntax for particular
fields.  This separation is intended to allow simple  parsers  to
operate on the general structure of messages, without concern for
the detailed structure of individual header fields.   Appendix  B
is provided to facilitate construction of these parsers.In addition to the fields specified in this document, it  is
expected  that  other fields will gain common use.  As necessary,
the specifications for these "extension-fields" will be published
through  the same mechanism used to publish this document.  Users
may also  wish  to  extend  the  set  of  fields  that  they  use
privately.  Such "user-defined fields" are permitted.The framework severely constrains document tone and  appear-
ance and is primarily useful for most intra-organization communi-
cations and  well-structured   inter-organization  communication.
It  also  can  be used for some types of inter-process communica-
tion, such as simple file transfer and remote job entry.  A  more
robust  framework might allow for multi-font, multi-color, multi-
dimension encoding of information.  A  less  robust  one,  as  is
present  in  most  single-machine  message  systems,  would  more
severely constrain the ability to add fields and the decision  to
include specific fields.  In contrast with paper-based communica-
tion, it is interesting to note that the RECEIVER  of  a  message
can   exercise  an  extraordinary  amount  of  control  over  the
message's appearance.  The amount of actual control available  to
message  receivers  is  contingent upon the capabilities of their
individual message systems.

2. Notational Conventions

This specification uses an augmented Backus-Naur Form (BNF) notation. The differences from standard BNF involve naming rules and indicating repetition and "local" alternatives.

2.1. RULE NAMING

Angle brackets ("<", ">") are not  used,  in  general.   The
name  of  a rule is simply the name itself, rather than "<name>".
Quotation-marks enclose literal text (which may be  upper  and/or
lower  case).   Certain  basic  rules  are  in uppercase, such as
SPACE, TAB, CRLF, DIGIT, ALPHA, etc.  Angle brackets are used  in
rule  definitions,  and  in  the rest of this  document, whenever
their presence will facilitate discerning the use of rule names.

2.2. RULE1 / RULE2: ALTERNATIVES

Elements separated by slash ("/") are alternatives.   There-
fore "foo / bar" will accept foo or bar.

2.3. (RULE1 RULE2): LOCAL ALTERNATIVES

Elements enclosed in parentheses are  treated  as  a  single
element.   Thus,  "(elem  (foo  /  bar)  elem)"  allows the token
sequences "elem foo elem" and "elem bar elem".

2.4. *RULE: REPETITION

The character "*" preceding an element indicates repetition.
The full form is:<l>*<m>elementindicating at least <l> and at most <m> occurrences  of  element.
Default values are 0 and infinity so that "*(element)" allows any
number, including zero; "1*element" requires at  least  one;  and
"1*2element" allows one or two.

2.5. [RULE]: OPTIONAL

Square brackets enclose optional elements; "[foo  bar]"   is
equivalent to "*1(foo bar)".

2.6. NRULE: SPECIFIC REPETITION

"<n>(element)" is equivalent to "<n>*<n>(element)"; that is,
exactly  <n>  occurrences  of (element). Thus 2DIGIT is a 2-digit
number, and 3ALPHA is a string of three alphabetic characters.

2.7. #RULE: LISTS

A construct "#" is defined, similar to "*", as follows:<l>#<m>elementindicating at least <l> and at most <m> elements, each  separated
by  one  or more commas (","). This makes the usual form of lists
very easy; a rule such as '(element *("," element))' can be shown
as  "1#element".   Wherever this construct is used, null elements
are allowed, but do not  contribute  to  the  count  of  elements
present.   That  is,  "(element),,(element)"  is  permitted,  but
counts as only two elements.  Therefore, where at least one  ele-
ment  is required, at least one non-null element must be present.
Default values are 0 and infinity so that "#(element)" allows any
number,  including  zero;  "1#element" requires at least one; and
"1#2element" allows one or two.

2.8. ; COMMENTS

A semi-colon, set off some distance to  the  right  of  rule
text,  starts  a comment that continues to the end of line.  This
is a simple way of including useful notes in  parallel  with  the
specifications.

3. LEXICAL ANALYSIS OF MESSAGES

3.1. GENERAL DESCRIPTION

A message consists of header fields and, optionally, a body. The body is simply a sequence of lines containing ASCII characters. It is separated from the headers by a null line (i.e., a line with nothing preceding the CRLF).

3.1.1. LONG HEADER FIELDS

Each header field can be viewed as a single, logical line of ASCII characters, comprising a field-name and a field-body. For convenience, the field-body portion of this conceptual entity can be split into a multiple-line representation; this is called "folding". The general rule is that wherever there may be linear-white-space (NOT simply LWSP-chars), a CRLF immediately followed by AT LEAST one LWSP-char may instead be inserted. Thus, the single line

To:  "Joe & J. Harvey" <ddd @Org>, JJV @ BBN

can be represented as:

To:  "Joe & J. Harvey" <ddd @ Org>,JJV@BBN

and

To:  "Joe & J. Harvey"<ddd@ Org>, JJV@BBN

and

To:  "Joe &J. Harvey" <ddd @ Org>, JJV @ BBN

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 regarding CRLF immediately followed by a LWSP-char as equivalent to the LWSP-char.

Note:

While the standard permits folding wherever linear-white-space is permitted, it is recommended that structured fields, such as those containing addresses, limit folding to higher-level syntactic breaks. For address fields, it is recommended that such folding occur between addresses, after the separating comma.

3.1.2. STRUCTURE OF HEADER FIELDS

Once a field has been unfolded, it may be viewed as being composed of a field-name followed by a colon (":"), followed by a field-body, and terminated by a carriage-return/line-feed. The field-name must be composed of printable ASCII characters (i.e., characters that have values between 33. and 126., decimal, except colon). The field-body may be composed of any ASCII characters, except CR or LF. (While CR and/or LF may be present in the actual text, they are removed by the action of unfolding the field.)

Certain field-bodies of headers may be interpreted according to an internal syntax that some systems may wish to parse. These fields are called "structured fields". Examples include fields containing dates and addresses. Other fields, such as "Subject" and "Comments", are regarded simply as strings of text.

Note:

Any field which has a field-body that is defined as other than simply <text> is to be treated as a structured field.

Field-names, unstructured field bodies and structured field bodies each are scanned by their own, independent "lexical" analyzers.

3.1.3. UNSTRUCTURED FIELD BODIES

For some fields, such as "Subject" and "Comments", no structuring is assumed, and they are treated simply as <text>s, as in the message body. Rules of folding apply to these fields, so that such field bodies which occupy several lines must therefore have the second and successive lines indented by at least one LWSP-char.

3.1.4. STRUCTURED FIELD BODIES

To aid in the creation and reading of structured fields, the free insertion of linear-white-space (which permits folding by inclusion of CRLFs) is allowed between lexical tokens. Rather than obscuring the syntax specifications for these structured fields with explicit syntax for this linear-white-space, the existence of another "lexical" analyzer is assumed. This analyzer does not apply for unstructured field bodies that are simply strings of text, as described above. The analyzer provides an interpretation of the unfolded text composing the body of the field as a sequence of lexical symbols.

These symbols are:

• individual special characters
• quoted-strings
• domain-literals
• comments
• atoms

The first four of these symbols are self-delimiting. Atoms are not; they are delimited by the self-delimiting symbols and by linear-white-space. For the purposes of regenerating sequences of atoms and quoted-strings, exactly one SPACE is assumed to exist, and should be used, between them. (Also, in the "Clarifications" section on "White Space", below, note the rules about treatment of multiple contiguous LWSP-chars.)

So, for example, the folded body of an address field

":sysmail"@  Some-Group. Some-Org,Muhammed.(I am  the greatest) Ali @(the)Vegas.WBA

is analyzed into the following lexical symbols and types:

:sysmail              quoted string@                     specialSome-Group            atom.                     specialSome-Org              atom,                     specialMuhammed              atom.                     special(I am  the greatest)  commentAli                   atom@                     atom(the)                 commentVegas                 atom.                     specialWBA                   atom

The canonical representations for the data in these addresses are the following strings:

":sysmail"@Some-Group.Some-OrgandMuhammed.Ali@Vegas.WBA

Note:

For purposes of display, and when passing such structured information to other systems, such as mail protocol services, there must be NO linear-white-space between <word>s that are separated by period (".") or at-sign ("@") and exactly one SPACE between all other <word>s. Also, headers should be in a folded form.

3.2. HEADER FIELD DEFINITIONS

These rules show a field meta-syntax, without regard for the particular type or internal syntax. Their purpose is to permit detection of fields; also, they present to higher-level parsers an image of each field as fitting on one line.

field       =  field-name ":" [ field-body ] CRLFfield-name  =  1*<any CHAR, excluding CTLs, SPACE, and ":">field-body  =  field-body-contents[CRLF LWSP-char field-body]field-body-contents =<the ASCII characters making up the field-body, asdefined in the following sections, and consistingof combinations of atom, quoted-string, andspecials tokens, or else consisting of texts>

3.3. LEXICAL TOKENS

The following rules are used to define an underlying lexical analyzer, which feeds tokens to higher level parsers. See the ANSI references, in the Bibliography.

; (  Octal, Decimal.)
CHAR        =  <any ASCII character>        ; (  0-177,  0.-127.)
ALPHA       =  <any ASCII alphabetic character>; (101-132, 65.- 90.); (141-172, 97.-122.)
DIGIT       =  <any ASCII decimal digit>    ; ( 60- 71, 48.- 57.)
CTL         =  <any ASCII control           ; (  0- 37,  0.- 31.)character and DEL>          ; (    177,     127.)
CR          =  <ASCII CR, carriage return>  ; (     15,      13.)
LF          =  <ASCII LF, linefeed>         ; (     12,      10.)
SPACE       =  <ASCII SP, space>            ; (     40,      32.)
HTAB        =  <ASCII HT, horizontal-tab>   ; (     11,       9.)
<">         =  <ASCII quote mark>           ; (     42,      34.)
CRLF        =  CR LFLWSP-char   =  SPACE / HTAB                 ; semantics = SPACElinear-white-space =  1*([CRLF] LWSP-char)  ; semantics = SPACE; CRLF => foldingspecials    =  "(" / ")" / "<" / ">" / "@"  ; Must be in quoted-/  "," / ";" / ":" / "\" / <">  ;  string, to use/  "." / "[" / "]"              ;  within a word.delimiters  =  specials / linear-white-space / commenttext        =  <any CHAR, including bare    ; => atoms, specials,CR & bare LF, but NOT       ;  comments andincluding CRLF>             ;  quoted-strings are;  NOT recognized. atom =  1*<any CHAR except specials, SPACE and CTLs>quoted-string = <"> *(qtext/quoted-pair) <">; Regular qtext or;   quoted chars.qtext       =  <any CHAR excepting <">,     ; => may be folded"\" & CR, and includinglinear-white-space>domain-literal =  "[" *(dtext / quoted-pair) "]"dtext       =  <any CHAR excluding "[",     ; => may be folded"]", "\" & CR, & includinglinear-white-space>comment     =  "(" *(ctext / quoted-pair / comment) ")"ctext       =  <any CHAR excluding "(",     ; => may be folded")", "\" & CR, & includinglinear-white-space>quoted-pair =  "\" CHAR                     ; may quote any charphrase      =  1*word                       ; Sequence of words word =  atom / quoted-string

3.4. CLARIFICATIONS

3.4.1. QUOTING

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. To quote a character, precede it with a backslash ("\").

This mechanism is not fully general. Characters may be quoted only within a subset of the lexical constructs. In particular, quoting is limited to use within:

• - quoted-string
• - domain-literal
• - comment

Within these constructs, quoting is REQUIRED for CR and "\" and for the character(s) that delimit the token (e.g., "(" and ")" for a comment). However, quoting is PERMITTED for any character.

Note:

In particular, quoting is NOT permitted within atoms. For example when the local-part of an addr-spec must contain a special character, a quoted string must be used. Therefore, a specification such as:

Full\ Name@Domain

is not legal and must be specified as:

"Full Name"@Domain

3.4.2. WHITE SPACE

Note:  In structured field bodies, multiple linear space ASCIIcharacters  (namely  HTABs  and  SPACEs) are treated assingle spaces and may freely surround any  symbol.   Inall header fields, the only place in which at least oneLWSP-char is REQUIRED is at the beginning of  continua-tion lines in a folded field.When passing text to processes  that  do  not  interpret  textaccording to this standard (e.g., mail protocol servers), thenNO linear-white-space characters should occur between a period(".") or at-sign ("@") and a <word>.  Exactly ONE SPACE shouldbe used in place of arbitrary linear-white-space  and  commentsequences.Note:  Within systems conforming to this standard, wherever  amember of the list of delimiters is allowed, LWSP-charsmay also occur before and/or after it.Writers of  mail-sending  (i.e.,  header-generating)  programsshould realize that there is no network-wide definition of theeffect of ASCII HT (horizontal-tab) characters on the  appear-ance  of  text  at another network host; therefore, the use oftabs in message headers, though permitted, is discouraged.

3.4.3. COMMENTS

A comment is a set of ASCII characters, which is  enclosed  inmatching  parentheses  and which is not within a quoted-stringThe comment construct permits message originators to add  textwhich  will  be  useful  for  human readers, but which will beignored by the formal semantics.  Comments should be  retainedwhile  the  message  is subject to interpretation according tothis standard.  However, comments  must  NOT  be  included  inother  cases,  such  as  during  protocol  exchanges with mailservers.Comments nest, so that if an unquoted left parenthesis  occursin  a  comment  string,  there  must  also be a matching rightparenthesis.  When a comment acts as the delimiter  between  asequence of two lexical symbols, such as two atoms, it is lex-ically equivalent with a single SPACE,  for  the  purposes  ofregenerating  the  sequence, such as when passing the sequenceonto a mail protocol server.  Comments are  detected  as  suchonly within field-bodies of structured fields.If a comment is to be "folded" onto multiple lines,  then  thesyntax  for  folding  must  be  adhered to.  (See the "LexicalAnalysis of Messages" section on "Folding Long Header  Fields"above,  and  the  section on "Case Independence" below.)  Notethat  the  official  semantics  therefore  do  not  "see"  anyunquoted CRLFs that are in comments, although particular pars-ing programs may wish to note their presence.  For these  pro-grams,  it would be reasonable to interpret a "CRLF LWSP-char"as being a CRLF that is part of the comment; i.e., the CRLF iskept  and  the  LWSP-char is discarded.  Quoted CRLFs (i.e., abackslash followed by a CR followed by a  LF)  still  must  befollowed by at least one LWSP-char.3.4.4.  DELIMITING AND QUOTING CHARACTERSThe quote character (backslash) and  characters  that  delimitsyntactic  units  are not, generally, to be taken as data thatare part of the delimited or quoted unit(s).   In  particular,the   quotation-marks   that   define   a  quoted-string,  theparentheses that define  a  comment  and  the  backslash  thatquotes  a  following  character  are  NOT  part of the quoted-string, comment or quoted character.  A quotation-mark that isto  be  part  of  a quoted-string, a parenthesis that is to bepart of a comment and a backslash that is to be part of eithermust  each be preceded by the quote-character backslash ("\").Note that the syntax allows any character to be quoted  withina  quoted-string  or  comment; however only certain charactersMUST be quoted to be included as data.  These  characters  arethe  ones that are not part of the alternate text group (i.e.,ctext or qtext).The one exception to this rule  is  that  a  single  SPACE  isassumed  to  exist  between  contiguous words in a phrase, andthis interpretation is independent of  the  actual  number  ofLWSP-chars  that  the  creator  places  between the words.  Toinclude more than one SPACE, the creator must make  the  LWSP-chars be part of a quoted-string.Quotation marks that delimit a quoted string  and  backslashesthat  quote  the  following character should NOT accompany thequoted-string when the string is passed to processes  that  donot interpret data according to this specification (e.g., mailprotocol servers).3.4.5.  QUOTED-STRINGSWhere permitted (i.e., in words in structured fields)  quoted-strings  are  treated  as a single symbol.  That is, a quoted-string is equivalent to an atom, syntactically.  If a  quoted-string  is to be "folded" onto multiple lines, then the syntaxfor folding must be adhered to.  (See the "Lexical Analysis ofMessages"  section  on "Folding Long Header Fields" above, andthe section on "Case  Independence"  below.)   Therefore,  theofficial  semantics  do  not  "see" any bare CRLFs that are inquoted-strings; however particular parsing programs  may  wishto  note  their presence.  For such programs, it would be rea-sonable to interpret a "CRLF LWSP-char" as being a CRLF  whichis  part  of the quoted-string; i.e., the CRLF is kept and theLWSP-char is discarded.  Quoted CRLFs (i.e., a backslash  fol-lowed  by  a CR followed by a LF) are also subject to rules offolding, but the presence of the quoting character (backslash)explicitly  indicates  that  the  CRLF  is  data to the quotedstring.  Stripping off the first following LWSP-char  is  alsoappropriate when parsing quoted CRLFs.

3.4.6. BRACKETING CHARACTERS

There is one type of bracket which must occur in matched pairsand may have pairs nested within each other:o   Parentheses ("(" and ")") are used  to  indicate  com-ments.There are three types of brackets which must occur in  matchedpairs, and which may NOT be nested:o   Colon/semi-colon (":" and ";") are   used  in  addressspecifications  to  indicate that the included list ofaddresses are to be treated as a group.o   Angle brackets ("<" and ">")  are  generally  used  toindicate  the  presence of a one machine-usable refer-ence (e.g., delimiting mailboxes), possibly  includingsource-routing to the machine.o   Square brackets ("[" and "]") are used to indicate thepresence  of  a  domain-literal, which the appropriatename-domain  is  to  use  directly,  bypassing  normalname-resolution mechanisms.

3.4.7. CASE INDEPENDENCE

Except as noted, alphabetic strings may be represented in any combination of upper and lower case. The only syntactic units which requires preservation of case information are:

• text
• qtext
• dtext
• ctext
• quoted-pair
• local-part, except "Postmaster"

When matching any other syntactic unit, case is to be ignored.For  example, the field-names "From", "FROM", "from", and even"FroM" are semantically equal and should all be treated ident-ically.When generating these units, any mix of upper and  lower  casealphabetic  characters  may  be  used.  The case shown in thisspecification is suggested for message-creating processes.Note:  The reserved local-part address unit, "Postmaster",  isan  exception.   When  the  value "Postmaster" is beinginterpreted, it must be  accepted  in  any  mixture  ofcase, including "POSTMASTER", and "postmaster".3.4.8.  FOLDING LONG HEADER FIELDSEach header field may be represented on exactly one line  con-sisting  of the name of the field and its body, and terminatedby a CRLF; this is what the parser sees.  For readability, thefield-body  portion of long header fields may be "folded" ontomultiple lines of the actual field.  "Long" is commonly inter-preted  to  mean greater than 65 or 72 characters.  The formerlength serves as a limit, when the message is to be viewed  onmost  simple terminals which use simple display software; how-ever, the limit is not imposed by this standard.Note:  Some display software often can selectively fold lines,to  suit  the display terminal.  In such cases, sender-provided  folding  can  interfere  with   the   displaysoftware.3.4.9.  BACKSPACE CHARACTERSASCII BS characters (Backspace, decimal 8) may be included  intexts and quoted-strings to effect overstriking.  However, anyuse of backspaces which effects an overstrike to the  left  ofthe beginning of the text or quoted-string is prohibited.

3.4.10. NETWORK-SPECIFIC TRANSFORMATIONS

During transmission through heterogeneous networks, it may  benecessary  to  force data to conform to a network's local con-ventions.  For example, it may be required that a CR  be  fol-lowed  either by LF, making a CRLF, or by <null>, if the CR isto stand alone).  Such transformations are reversed, when  themessage exits that network.When  crossing  network  boundaries,  the  message  should  betreated  as  passing  through  two modules.  It will enter thefirst module containing whatever network-specific  transforma-tions  that  were  necessary  to  permit migration through the"current" network.  It then passes through the modules:

Transformation ReversalThe "current" network's idiosyncracies are removed and the message is returned to the canonical form specified in this standard.TransformationThe "next" network's local idiosyncracies are imposed on the message.

------------------From   ==>  | Remove Net-A   |Net-A       | idiosyncracies |------------------||\/Conformancewith standard||\/------------------| Impose Net-B   |  ==>  To| idiosyncracies |       Net-B------------------

4. Message Specification

4.1. SYNTAX

Note:  Due to an artifact of the notational conventions, the syn-tax  indicates that, when present, some fields, must be ina particular order.  Header fields  are  NOT  required  tooccur  in  any  particular  order, except that the messagebody must occur AFTER  the  headers.   It  is  recommendedthat,  if  present,  headers be sent in the order "Return-Path", "Received", "Date",  "From",  "Subject",  "Sender","To", "cc", etc.This specification permits multiple  occurrences  of  mostfields.   Except  as  noted,  their  interpretation is notspecified here, and their use is discouraged.The following syntax for the bodies of various fields should
be  thought  of  as  describing  each field body as a single long
string (or line).  The "Lexical Analysis of Message"  section  on
"Long  Header Fields", above, indicates how such long strings can
be represented on more than one line in  the  actual  transmitted
message.message     =  fields *( CRLF *text )       ; Everything after;  first null line;  is message bodyfields      =    dates                      ; Creation time,source                     ;  author id & one1*destination                ;  address required*optional-field             ;  others optionalsource      = [  trace ]                    ; net traversalsoriginator                 ; original mail[  resent ]                   ; forwardedtrace       =    return                     ; path to sender1*received                   ; receipt tagsreturn      =  "Return-path" ":" route-addr ; return addressreceived    =  "Received"    ":"            ; one per relay["from" domain]           ; sending host["by"   domain]           ; receiving host["via"  atom]             ; physical path*("with" atom)             ; link/mail protocol["id"   msg-id]           ; receiver msg id["for"  addr-spec]        ; initial form";"    date-time         ; time receivedoriginator  =   authentic                   ; authenticated addr[ "Reply-To"   ":" 1#address] )authentic   =   "From"       ":"   mailbox  ; Single author/ ( "Sender"     ":"   mailbox  ; Actual submittor"From"       ":" 1#mailbox) ; Multiple authors;  or not senderresent      =   resent-authentic[ "Resent-Reply-To"  ":" 1#address] )resent-authentic ==   "Resent-From"      ":"   mailbox/ ( "Resent-Sender"    ":"   mailbox"Resent-From"      ":" 1#mailbox  )dates       =   orig-date                   ; Original[ resent-date ]               ; Forwardedorig-date   =  "Date"        ":"   date-timeresent-date =  "Resent-Date" ":"   date-timedestination =  "To"          ":" 1#address  ; Primary/  "Resent-To"   ":" 1#address/  "cc"          ":" 1#address  ; Secondary/  "Resent-cc"   ":" 1#address/  "bcc"         ":"  #address  ; Blind carbon/  "Resent-bcc"  ":"  #addressoptional-field =/  "Message-ID"        ":"   msg-id/  "Resent-Message-ID" ":"   msg-id/  "In-Reply-To"       ":"  *(phrase / msg-id)/  "References"        ":"  *(phrase / msg-id)/  "Keywords"          ":"  #phrase/  "Subject"           ":"  *text/  "Comments"          ":"  *text/  "Encrypted"         ":" 1#2word/  extension-field              ; To be defined/  user-defined-field           ; May be pre-empted msg-id =  "<" addr-spec ">"            ; Unique message idextension-field =<Any field which is defined in a documentpublished as a formal extension to thisspecification; none will have names beginningwith the string "X-">user-defined-field =<Any field which has not been definedin this specification or published as anextension to this specification; names forsuch fields must be unique and may bepre-empted by published extensions>

4.2. FORWARDING

Some systems permit mail recipients to  forward  a  message,
retaining  the original headers, by adding some new fields.  This
standard supports such a service, through the "Resent-" prefix to
field names.Whenever the string "Resent-" begins a field name, the field
has  the  same  semantics as a field whose name does not have the
prefix.  However, the message is assumed to have  been  forwarded
by  an original recipient who attached the "Resent-" field.  This
new field is treated as being more recent  than  the  equivalent,
original  field.   For  example, the "Resent-From", indicates the
person that forwarded the message, whereas the "From" field indi-
cates the original author.Use of such precedence  information  depends  upon  partici-
pants'  communication needs.  For example, this standard does not
dictate when a "Resent-From:" address should receive replies,  in
lieu of sending them to the "From:" address.Note:  In general, the "Resent-" fields should be treated as con-taining  a  set  of information that is independent of theset of original fields.  Information for  one  set  shouldnot  automatically be taken from the other.  The interpre-tation of multiple "Resent-" fields, of the same type,  isundefined.

In the remainder of this specification, occurrence of legal "Resent-" fields are treated identically with the occurrence of fields whose names do not contain this prefix.

4.3. TRACE FIELDS

Trace information is used to provide an audit trail of message handling. In addition, it indicates a route back to the sender of the message.

The list of known "via" and "with" values are registered with the Network Information Center, SRI International, Menlo Park, California.

4.3.1. RETURN-PATH

This field  is  added  by  the  final  transport  system  thatdelivers  the message to its recipient.  The field is intendedto contain definitive information about the address and  routeback to the message's originator.Note:  The "Reply-To" field is added  by  the  originator  andserves  to  direct  replies,  whereas the "Return-Path"field is used to identify a path back to  the  origina-tor.While the syntax  indicates  that  a  route  specification  isoptional,  every attempt should be made to provide that infor-mation in this field.

4.3.2. RECEIVED

A copy of this field is added by each transport  service  thatrelays the message.  The information in the field can be quiteuseful for tracing transport problems.The names of the sending  and  receiving  hosts  and  time-of-receipt may be specified.  The "via" parameter may be used, toindicate what physical mechanism the message  was  sent  over,such  as  Arpanet or Phonenet, and the "with" parameter may beused to indicate the mail-,  or  connection-,  level  protocolthat  was  used, such as the SMTP mail protocol, or X.25 tran-sport protocol.Note:  Several "with" parameters may  be  included,  to  fullyspecify the set of protocols that were used.Some transport services queue mail; the internal message iden-tifier that is assigned to the message may be noted, using the"id" parameter.  When the  sending  host  uses  a  destinationaddress specification that the receiving host reinterprets, byexpansion or transformation, the receiving host  may  wish  torecord  the original specification, using the "for" parameter.For example, when a copy of mail is sent to the  member  of  adistribution  list,  this  parameter may be used to record theoriginal address that was used to specify the list.

4.4. ORIGINATOR FIELDS

The standard allows only a subset of the combinations possi-
ble  with the From, Sender, Reply-To, Resent-From, Resent-Sender,
and Resent-Reply-To fields.  The limitation is intentional.

4.4.1. FROM / RESENT-FROM

This field contains the identity of the person(s)  who  wishedthis  message to be sent.  The message-creation process shoulddefault this field  to  be  a  single,  authenticated  machineaddress,  indicating  the  AGENT  (person,  system or process)entering the message.  If this is not done, the "Sender" fieldMUST  be  present.  If the "From" field IS defaulted this way,the "Sender" field is  optional  and  is  redundant  with  the"From"  field.   In  all  cases, addresses in the "From" fieldmust be machine-usable (addr-specs) and may not contain  namedlists (groups).

4.4.2. SENDER / RESENT-SENDER

This field contains the authenticated identity  of  the  AGENT(person,  system  or  process)  that sends the message.  It isintended for use when the sender is not the author of the mes-sage,  or  to  indicate  who among a group of authors actuallysent the message.  If the contents of the "Sender" field wouldbe  completely  redundant  with  the  "From"  field,  then the"Sender" field need not be present and its use is  discouraged(though  still legal).  In particular, the "Sender" field MUSTbe present if it is NOT the same as the "From" Field.The Sender mailbox  specification  includes  a  word  sequencewhich  must correspond to a specific agent (i.e., a human useror a computer program) rather than a standard  address.   Thisindicates  the  expectation  that  the field will identify thesingle AGENT (person,  system,  or  process)  responsible  forsending  the mail and not simply include the name of a mailboxfrom which the mail was sent.  For example in the  case  of  ashared login name, the name, by itself, would not be adequate.The local-part address unit, which refers to  this  agent,  isexpected to be a computer system term, and not (for example) ageneralized person reference which can  be  used  outside  thenetwork text message context.Since the critical function served by the  "Sender"  field  isidentification  of  the agent responsible for sending mail andsince computer programs cannot be held accountable  for  theirbehavior, it is strongly recommended that when a computer pro-gram generates a message, the HUMAN  who  is  responsible  forthat program be referenced as part of the "Sender" field mail-box specification.

4.4.3. REPLY-TO / RESENT-REPLY-TO

This field provides a general  mechanism  for  indicating  anymailbox(es)  to which responses are to be sent.  Three typicaluses for this feature can  be  distinguished.   In  the  firstcase,  the  author(s) may not have regular machine-based mail-boxes and therefore wish(es) to indicate an alternate  machineaddress.   In  the  second case, an author may wish additionalpersons to be made aware of, or responsible for,  replies.   Asomewhat  different  use  may be of some help to "text messageteleconferencing" groups equipped with automatic  distributionservices:   include the address of that service in the "Reply-To" field of all messages  submitted  to  the  teleconference;then  participants  can  "reply"  to conference submissions toguarantee the correct distribution of any submission of  theirown.Note:  The "Return-Path" field is added by the mail  transportservice,  at the time of final deliver.  It is intendedto identify a path back to the orginator  of  the  mes-sage.   The  "Reply-To"  field  is added by the messageoriginator and is intended to direct replies.4.4.4.  AUTOMATIC USE OF FROM / SENDER / REPLY-TOFor systems which automatically  generate  address  lists  forreplies to messages, the following recommendations are made:o   The "Sender" field mailbox should be sent  notices  ofany  problems in transport or delivery of the originalmessages.  If there is no  "Sender"  field,  then  the"From" field mailbox should be used.o   The  "Sender"  field  mailbox  should  NEVER  be  usedautomatically, in a recipient's reply message.o   If the "Reply-To" field exists, then the reply  shouldgo to the addresses indicated in that field and not tothe address(es) indicated in the "From" field.o   If there is a "From" field, but no  "Reply-To"  field,the  reply should be sent to the address(es) indicatedin the "From" field.Sometimes, a recipient may actually wish to  communicate  withthe  person  that  initiated  the  message  transfer.  In suchcases, it is reasonable to use the "Sender" address.This recommendation is intended  only  for  automated  use  oforiginator-fields  and is not intended to suggest that repliesmay not also be sent to other recipients of messages.   It  isup  to  the  respective  mail-handling programs to decide whatadditional facilities will be provided.Examples are provided in Appendix A.4.5.  RECEIVER FIELDS4.5.1.  TO / RESENT-TOThis field contains the identity of the primary recipients  ofthe message.4.5.2.  CC / RESENT-CCThis field contains the identity of  the  secondary  (informa-tional) recipients of the message.4.5.3.  BCC / RESENT-BCCThis field contains the identity of additional  recipients  ofthe  message.   The contents of this field are not included incopies of the message sent to the primary and secondary  reci-pients.   Some  systems  may choose to include the text of the"Bcc" field only in the author(s)'s  copy,  while  others  mayalso include it in the text sent to all those indicated in the"Bcc" list.

4.6. REFERENCE FIELDS

4.6.1. Message-ID / Resent-Message-ID

This field contains a  unique identifier  (the local-part address unit) which refers to THIS version of THIS message. The uniqueness of the message identifier is guaranteed by the host which generates it. 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.

4.6.2. IN-REPLY-TO

The contents of this field identify previous correspondence which this message answers. Note that if message identifiers are used in this field, they must use the  msg-id specification format.

4.6.3. REFERENCES

The contents of this field identify other correspondence which this message references. Note that if message identifiers are used, they must use the  msg-id  specification format.

4.6.4. KEYWORDS

This field contains keywords or phrases, separated by commas.

4.7. Other Fields

4.7.1. SUBJECT

This is intended to provide a summary, or indicate the nature, of the message.

4.7.2. COMMENTS

Permits adding text comments onto the message without disturbing the contents of the message's body.

4.7.3. ENCRYPTED

Sometimes,  data  encryption  is  used  to  increase  theprivacy  of  message  contents.   If the body of a message hasbeen encrypted, to keep its contents private, the  "Encrypted"field  can be used to note the fact and to indicate the natureof the encryption.  The first <word> parameter  indicates  thesoftware  used  to  encrypt the body, and the second, optional<word> is intended to  aid  the  recipient  in  selecting  theproper  decryption  key.   This  code word may be viewed as anindex to a table of keys held by the recipient.Note:  Unfortunately, headers must contain envelope,  as  wellas  contents,  information.  Consequently, it is neces-sary that they remain unencrypted, so that  mail  tran-sport   services   may   access   them.   Since  names,addresses, and "Subject"  field  contents  may  containsensitive  information,  this  requirement limits totalmessage privacy.Names of encryption software are registered with the Net-work  Information Center, SRI International, Menlo Park, Cali-fornia.

4.7.4. EXTENSION-FIELD

A limited number of common fields have  been  defined  inthis  document.   As  network mail requirements dictate, addi-tional fields may be standardized.   To  provide  user-definedfields  with  a  measure  of  safety,  in name selection, suchextension-fields will never have names  that  begin  with  thestring "X-".Names of Extension-fields are registered with the NetworkInformation Center, SRI International, Menlo Park, California.

4.7.5. USER-DEFINED-FIELD

Individual users of network mail are free to  define  anduse  additional  header  fields.   Such fields must have nameswhich are not already used in the current specification or  inany definitions of extension-fields, and the overall syntax ofthese user-defined-fields must conform to this specification'srules   for   delimiting  and  folding  fields.   Due  to  theextension-field  publishing  process,  the  name  of  a  user-defined-field may be pre-emptedNote:  The prefatory string "X-" will never  be  used  in  thenames  of Extension-fields.  This provides user-definedfields with a protected set of names.

5. Date and Time Specification

5.1. SYNTAX

date-time   =  [ day "," ] date time        ; dd mm yy;  hh:mm:ss zzzday         =  "Mon"  / "Tue" /  "Wed"  / "Thu"/  "Fri"  / "Sat" /  "Sun"date        =  1*2DIGIT month 2DIGIT        ; day month year;  e.g. 20 Jun 82month       =  "Jan"  /  "Feb" /  "Mar"  /  "Apr"/  "May"  /  "Jun" /  "Jul"  /  "Aug"/  "Sep"  /  "Oct" /  "Nov"  /  "Dec"time        =  hour zone                    ; ANSI and Militaryhour        =  2DIGIT ":" 2DIGIT [":" 2DIGIT]; 00:00:00 - 23:59:59zone        =  "UT"  / "GMT"                ; Universal Time; North American : UT/  "EST" / "EDT"                ;  Eastern:  - 5/ - 4/  "CST" / "CDT"                ;  Central:  - 6/ - 5/  "MST" / "MDT"                ;  Mountain: - 7/ - 6/  "PST" / "PDT"                ;  Pacific:  - 8/ - 7/  1ALPHA                       ; Military: Z = UT;;  A:-1; (J not used);  M:-12; N:+1; Y:+12/ ( ("+" / "-") 4DIGIT )        ; Local differential;  hours+min. (HHMM)

5.2. SEMANTICS

If included, day-of-week must be the day implied by the date specification.

Time zone may be indicated in several ways. "UT" is Universal Time (formerly called "Greenwich Mean Time"); "GMT" is permitted as a reference to Universal Time. The military standard uses a single character for each zone. "Z" is Universal Time. "A" indicates one hour earlier, and "M" indicates 12 hours earlier; "N" is one hour later, and "Y" is 12 hours later. The letter "J" is not used. The other remaining two forms are taken from ANSI standard X3.51-1975. One allows explicit indication of the amount of offset from UT; the other uses common 3-character strings for indicating time zones in North America.

6. Address Specification

6.1. Syntax

address     = mailbox ; one addressee/ group ; named list group =  phrase ":" [#mailbox] ";"mailbox = addr-spec ; simple address/  phrase route-addr ; name & addr-specroute-addr =  "<" [route] addr-spec ">"route =  1#("@" domain) ":"           ; path-relativeaddr-spec = local-part "@" domain ; global addresslocal-part = word *("." word)             ; uninterpreted; case-preserveddomain = sub-domain *("." sub-domain)sub-domain = domain-ref / domain-literaldomain-ref = atom ; symbolic reference

6.2. SEMANTICS

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 their line printer and deliver the output to the addressee's desk.

A mailbox specification comprises a person, system or process name reference, a domain-dependent string, and a name-domain reference. The name reference is optional and is usually used to indicate the human name of a recipient. The name-domain reference specifies a sequence of sub-domains. The domain-dependent string is uninterpreted, except by the final sub-domain; the rest of the mail service merely transmits it as a literal string.

6.2.1. DOMAINS

A name-domain is a set of registered (mail)  names.   A  name-domain  specification  resolves  to  a subordinate name-domainspecification  or  to  a  terminal  domain-dependent   string.Hence,  domain  specification  is  extensible,  permitting anynumber of registration levels.Name-domains model a global, logical, hierarchical  addressingscheme.   The  model is logical, in that an address specifica-tion is related to name registration and  is  not  necessarilytied  to  transmission  path.   The  model's  hierarchy  is  adirected graph, called an in-tree, such that there is a singlepath  from  the root of the tree to any node in the hierarchy.If more than one path actually exists, they are considered  tobe different addresses.The root node is common to all addresses; consequently, it  isnot  referenced.   Its  children  constitute "top-level" name-domains.  Usually, a service has access to its own full domainspecification and to the names of all top-level name-domains.The "top" of the domain addressing hierarchy -- a child of theroot  --  is  indicated  by  the right-most field, in a domainspecification.  Its child is specified to the left, its  childto the left, and so on.Some groups provide formal registration services;  these  con-stitute   name-domains   that  are  independent  logically  ofspecific machines.  In addition, networks and machines  impli-citly  compose name-domains, since their membership usually isregistered in name tables.In the case of formal registration, an organization implementsa  (distributed)  data base which provides an address-to-routemapping service for addresses of the form:person@registry.organizationNote that "organization" is a logical  entity,  separate  fromany particular communication network.A mechanism for accessing "organization" is universally avail-able.   That mechanism, in turn, seeks an instantiation of theregistry; its location is not indicated in the address specif-ication.   It  is assumed that the system which operates underthe name "organization" knows how to find a subordinate regis-try.  The registry will then use the "person" string to deter-mine where to send the mail specification.The latter,  network-oriented  case  permits  simple,  direct,attachment-related address specification, such as:user@host.networkOnce the network is accessed, it is expected  that  a  messagewill  go  directly  to the host and that the host will resolvethe user name, placing the message in the user's mailbox.

6.2.2. ABBREVIATED DOMAIN SPECIFICATION

Since any number of  levels  is  possible  within  the  domainhierarchy,  specification  of  a  fully  qualified address canbecome inconvenient.  This standard permits abbreviated domainspecification, in a special case:For the address of  the  sender,  call  the  left-mostsub-domain  Level  N.   In a header address, if all ofthe sub-domains above (i.e., to the right of) Level  Nare  the same as those of the sender, then they do nothave to appear in the specification.   Otherwise,  theaddress must be fully qualified.This feature is subject  to  approval  by  local  sub-domains.   Individual  sub-domains  may  require theirmember systems, which originate mail, to provide  fulldomain  specification only.  When permitted, abbrevia-tions may be present  only  while  the  message  stayswithin the sub-domain of the sender.Use of this mechanism requires the sender's sub-domainto reserve the names of all top-level domains, so thatfull specifications can be distinguished from abbrevi-ated specifications.For example, if a sender's address is:sender@registry-A.registry-1.organization-Xand one recipient's address is:recipient@registry-B.registry-1.organization-Xand another's is:recipient@registry-C.registry-2.organization-Xthen ".registry-1.organization-X" need not be specified in thethe  message,  but  "registry-C.registry-2"  DOES  have  to bespecified.  That is, the first two addresses may  be  abbrevi-ated, but the third address must be fully specified.When a message crosses a domain boundary, all  addresses  mustbe  specified  in  the  full format, ending with the top-levelname-domain in the right-most field.  It is the responsibilityof  mail  forwarding services to ensure that addresses conformwith this requirement.  In the case of abbreviated  addresses,the  relaying  service must make the necessary expansions.  Itshould be noted that it often is difficult for such a  serviceto locate all occurrences of address abbreviations.  For exam-ple, it will not be possible to find such abbreviations withinthe  body  of  the  message.   The "Return-Path" field can aidrecipients in recovering from these errors.Note:  When passing any portion of an addr-spec onto a processwhich  does  not interpret data according to this stan-dard (e.g., mail protocol servers).  There must  be  NOLWSP-chars  preceding  or  following the at-sign or anydelimiting period ("."), such as  shown  in  the  aboveexamples,   and   only  ONE  SPACE  between  contiguous<word>s.

6.2.3. DOMAIN TERMS

A domain-ref must be THE official name of a registry, network,or  host.   It  is  a  symbolic  reference, within a name sub-domain.  At times, it is necessary to bypass standard  mechan-isms  for  resolving  such  references,  using  more primitiveinformation, such as a network host address  rather  than  itsassociated host name.To permit such references, this standard provides the  domain-literal  construct.   Its contents must conform with the needsof the sub-domain in which it is interpreted.Domain-literals which refer to domains within the ARPA  Inter-net  specify  32-bit  Internet addresses, in four 8-bit fieldsnoted in decimal, as described in Request for  Comments  #820,"Assigned Numbers."  For example:[10.0.3.19]Note:  THE USE OF DOMAIN-LITERALS IS STRONGLY DISCOURAGED.  Itis  permitted  only  as  a means of bypassing temporarysystem limitations, such as name tables which  are  notcomplete.The names of "top-level" domains, and  the  names  of  domainsunder  in  the  ARPA Internet, are registered with the NetworkInformation Center, SRI International, Menlo Park, California.

6.2.4. DOMAIN-DEPENDENT LOCAL STRING

The local-part of an  addr-spec  in  a  mailbox  specification(i.e.,  the  host's  name for the mailbox) is understood to bewhatever the receiving mail protocol server allows.  For exam-ple,  some systems do not understand mailbox references of theform "P. D. Q. Bach", but others do.This specification treats periods (".") as lexical separators.Hence,  their  presence  in  local-parts which are not quoted-strings, is detected.   However,  such  occurrences  carry  NOsemantics.  That is, if a local-part has periods within it, anaddress parser will divide the local-part into several tokens,but  the  sequence  of  tokens will be treated as one uninter-preted unit.  The sequence  will  be  re-assembled,  when  theaddress is passed outside of the system such as to a mail pro-tocol service.For example, the address:First.Last@Registry.Orgis legal and does not require the local-part to be  surroundedwith  quotation-marks.   (However,  "First  Last" DOES requirequoting.)  The local-part of the address, when passed  outsideof  the  mail  system,  within  the  Registry.Org  domain,  is"First.Last", again without quotation marks.

6.2.5. BALANCING LOCAL-PART AND DOMAIN

In some cases, the boundary between local-part and domain  canbe  flexible.  The local-part may be a simple string, which isused for the final determination of the  recipient's  mailbox.All  other  levels  of  reference  are, therefore, part of thedomain.For some systems, in the case of abbreviated reference to  thelocal  and  subordinate  sub-domains,  it  may  be possible tospecify only one reference within the domain  part  and  placethe  other,  subordinate  name-domain  references  within  thelocal-part.  This would appear as:mailbox.sub1.sub2@this-domainSuch a specification would be acceptable  to  address  parserswhich  conform  to  RFC  #733,  but  do not support this newerInternet standard.  While contrary to the intent of this stan-dard, the form is legal.Also, some sub-domains have a specification syntax which  doesnot conform to this standard.  For example:sub-net.mailbox@sub-domain.domainuses a different parsing  sequence  for  local-part  than  fordomain.Note:  As a rule,  the  domain  specification  should  containfields  which  are  encoded  according to the syntax ofthis standard and which contain  generally-standardizedinformation.   The local-part specification should con-tain only that portion of the  address  which  deviatesfrom the form or intention of the domain field.

6.2.6. MULTIPLE MAILBOXES

An individual may have several mailboxes and wish  to  receivemail  at  whatever  mailbox  is  convenient  for the sender toaccess.  This standard does not provide a means of  specifying"any member of" a list of mailboxes.A set of individuals may wish to receive mail as a single unit(i.e.,  a  distribution  list).  The <group> construct permitsspecification of such a list.  Recipient mailboxes are  speci-fied  within  the  bracketed  part (":" - ";").  A copy of thetransmitted message is to be  sent  to  each  mailbox  listed.This  standard  does  not  permit  recursive  specification ofgroups within groups.While a list must be named, it is not required that  the  con-tents  of  the  list be included.  In this case, the <address>serves only as an indication of group distribution  and  wouldappear in the form:name:;Some mail  services  may  provide  a  group-list  distributionfacility,  accepting  a single mailbox reference, expanding itto the full distribution list, and relaying the  mail  to  thelist's  members.   This standard provides no additional syntaxfor indicating such a  service.   Using  the  <group>  addressalternative,  while listing one mailbox in it, can mean eitherthat the mailbox reference will be expanded to a list or  thatthere is a group with one member.

6.2.7. EXPLICIT PATH SPECIFICATION

At times, a  message  originator  may  wish  to  indicate  thetransmission  path  that  a  message  should  follow.  This iscalled source routing.  The normal addressing scheme, used  inan  addr-spec,  is  carefully separated from such information;the <route> portion of a route-addr is provided for such occa-sions.  It specifies the sequence of hosts and/or transmissionservices that are  to  be  traversed.   Both  domain-refs  anddomain-literals may be used.Note:  The use of source routing is discouraged.   Unless  thesender has special need of path restriction, the choiceof transmission route should be left to the mail  tran-sport service.

6.3. RESERVED ADDRESS

It often is necessary to send mail to a site, without knowing any of its valid addresses. For example, there may be mail system dysfunctions, or a user may wish to find out a person's correct address, at that site.

This standard specifies a single, reserved mailbox address (local-part) which is to be valid at each site. Mail sent to that address is to be routed to a person responsible for the site's mail system or to a person with responsibility for general site operation. The name of the reserved local-part address is:

Postmaster

so that "Postmaster@domain" is required to be valid.

Note: This reserved local-part must be matched without sensitivity to alphabetic case, so that "POSTMASTER", "postmaster", and even "poStmASteR" is to be accepted.

7. Bibliography

ANSI.  "USA Standard Code  for  Information  Interchange,"  X3.4.American  National Standards Institute: New York (1968).  Alsoin:  Feinler, E.  and J. Postel, eds., "ARPANET Protocol Hand-book", NIC 7104.ANSI.  "Representations of Universal Time, Local  Time  Differen-tials,  and United States Time Zone References for InformationInterchange," X3.51-1975.  American National Standards  Insti-tute:  New York (1975).Bemer, R.W., "Time and the Computer."  In:  Interface  Age  (Feb.1979).Bennett, C.J.  "JNT Mail Protocol".  Joint Network Team,  Ruther-ford and Appleton Laboratory:  Didcot, England.Bhushan, A.K., Pogran, K.T., Tomlinson,  R.S.,  and  White,  J.E."Standardizing  Network  Mail  Headers,"   ARPANET Request forComments No. 561, Network Information Center  No.  18516;  SRIInternational:  Menlo Park (September 1973).Birrell, A.D., Levin, R.,  Needham,  R.M.,  and  Schroeder,  M.D."Grapevine:  An Exercise in Distributed Computing," Communica-tions of the ACM 25, 4 (April 1982), 260-274.Crocker,  D.H.,  Vittal,  J.J.,  Pogran,  K.T.,  Henderson,  D.A."Standard  for  the  Format  of  ARPA  Network  Text Message,"ARPANET Request for  Comments  No.  733,  Network  InformationCenter  No.  41952.   SRI International:  Menlo Park (November1977).Feinler, E.J. and Postel, J.B.  ARPANET Protocol  Handbook,  Net-work  Information  Center  No.  7104   (NTIS AD A003890).  SRIInternational:  Menlo Park (April 1976).Harary, F.   "Graph  Theory".   Addison-Wesley:   Reading,  Mass.(1969).Levin, R. and Schroeder, M.  "Transport  of  Electronic  Messagesthrough  a  Network,"   TeleInformatics  79, pp. 29-33.  NorthHolland (1979).  Also  as  Xerox  Palo  Alto  Research  CenterTechnical Report CSL-79-4.Myer, T.H. and Henderson, D.A.  "Message Transmission  Protocol,"ARPANET  Request  for  Comments,  No. 680, Network InformationCenter No. 32116.  SRI International:  Menlo Park (1975).NBS.  "Specification of Message Format for Computer Based MessageSystems, Recommended Federal Information Processing Standard."National  Bureau   of   Standards:    Gaithersburg,   Maryland(October 1981).NIC.  Internet Protocol Transition Workbook.  Network InformationCenter,   SRI-International,  Menlo  Park,  California  (March1982).Oppen, D.C. and Dalal, Y.K.  "The Clearinghouse:  A DecentralizedAgent  for  Locating  Named  Objects in a Distributed Environ-ment," OPD-T8103.  Xerox Office Products Division:  Palo Alto,CA. (October 1981).Postel, J.B.  "Assigned Numbers,"  ARPANET Request for  Comments,No. 820.  SRI International:  Menlo Park (August 1982).Postel, J.B.  "Simple Mail Transfer  Protocol,"  ARPANET  Requestfor Comments, No. 821.  SRI International:  Menlo Park (August1982).Shoch, J.F.  "Internetwork naming, addressing  and  routing,"  inProc. 17th IEEE Computer Society International Conference, pp.72-79, Sept. 1978, IEEE Cat. No. 78 CH 1388-8C.Su, Z. and Postel, J.  "The Domain Naming Convention for InternetUser  Applications,"  ARPANET  Request  for Comments, No. 819.SRI International:  Menlo Park (August 1982).

APPENDIX A. EXAMPLES

A.1. ADDRESSES

A.1.1. Alfred Neuman <Neuman@BBN-TENEXA>

A.1.2. Neuman@BBN-TENEXA

These two "Alfred Neuman" examples have identical  seman-tics, as far as the operation of the local host's mail sending(distribution) program (also sometimes  called  its  "mailer")and  the remote host's mail protocol server are concerned.  Inthe first example, the  "Alfred  Neuman"  is  ignored  by  themailer,  as "Neuman@BBN-TENEXA" completely specifies the reci-pient.  The second example contains  no  superfluous  informa-tion,  and,  again,  "Neuman@BBN-TENEXA" is the intended reci-pient.Note:  When the message crosses name-domain  boundaries,  thenthese specifications must be changed, so as to indicatethe remainder of the hierarchy, starting with  the  toplevel.

A.1.3. "George, Ted" <Shared@Group.Arpanet>

This form might be used to indicate that a single mailboxis  shared  by several users.  The quoted string is ignored bythe originating host's mailer, because  "Shared@Group.Arpanet"completely specifies the destination mailbox.

A.1.4. Wilt . (the Stilt) Chamberlain@NBA.US

The "(the  Stilt)" is a comment, which is NOT included inthe  destination  mailbox  address  handed  to the originatingsystem's mailer.  The local-part of the address is the  string"Wilt.Chamberlain", with NO space between the first and secondwords.

A.1.5. Address Lists

Gourmets:  Pompous Person <WhoZiWhatZit@Cordon-Bleu>,Childs@WGBH.Boston, Galloping Gourmet@ANT.Down-Under (Australian National Television),Cheapie@Discount-Liquors;,Cruisers:  Port@Portugal, Jones@SEA;,Another@Somewhere.SomeOrgThis group list example points out the use of comments and themixing of addresses and groups.

A.2. ORIGINATOR ITEMS

A.2.1. Author-sent

George Jones logs into his host  as  "Jones".   He  sendsmail himself.From:  Jones@Group.OrgorFrom:  George Jones <Jones@Group.Org>

A.2.2. Secretary-sent

George Jones logs in as Jones on his  host.   His  secre-tary,  who logs in as Secy sends mail for him.  Replies to themail should go to George.From:    George Jones <Jones@Group>Sender:  Secy@Other-Group

A.2.3. Secretary-sent, for user of shared directory

George Jones' secretary sends mail  for  George.  Repliesshould go to George.From:     George Jones<Shared@Group.Org>Sender:   Secy@Other-GroupNote that there need not be a space between  "Jones"  and  the"<",  but  adding a space enhances readability (as is the casein other examples.

A.2.4. Committee activity, with one author

George is a member of a committee.  He wishes to have anyreplies to his message go to all committee members.From:     George Jones <Jones@Host.Net>Sender:   Jones@HostReply-To: The Committee: Jones@Host.Net,Smith@Other.Org,Doe@Somewhere-Else;Note  that  if  George  had  not  included  himself   in   theenumeration  of  The  Committee,  he  would not have gotten animplicit reply; the presence of the  "Reply-to"  field  SUPER-SEDES the sending of a reply to the person named in the "From"field.

A.2.5. Secretary acting as full agent of author

George Jones asks his secretary  (Secy@Host)  to  send  amessage for him in his capacity as Group.  He wants his secre-tary to handle all replies.From:     George Jones <Group@Host>Sender:   Secy@HostReply-To: Secy@Host

A.2.6. Agent for user without online mailbox

A friend  of  George's,  Sarah,  is  visiting.   George'ssecretary  sends  some  mail to a friend of Sarah in computer-land.  Replies should go to George, whose mailbox is Jones  atRegistry.From:     Sarah Friendly <Secy@Registry>Sender:   Secy-Name <Secy@Registry>Reply-To: Jones@Registry.

A.2.7. Agent for member of a committee

George's secretary sends out a message which was authoredjointly by all the members of a committee.  Note that the nameof the committee cannot be specified, since <group> names  arenot permitted in the From field.From:   Jones@Host,Smith@Other-Host,Doe@Somewhere-ElseSender: Secy@SHost

A.3. COMPLETE HEADERS

A.3.1. Minimum required

Date:     26 Aug 76 1429 EDT        Date:     26 Aug 76 1429 EDT
From:     Jones@Registry.Org   or   From:     Jones@Registry.Org
Bcc:                                To:       Smith@Registry.OrgNote that the "Bcc" field may be empty, while the  "To"  fieldis required to have at least one address.

A.3.2. Using some of the additional fields

Date:     26 Aug 76 1430 EDT
From:     George Jones<Group@Host>
Sender:   Secy@SHOST
To:       "Al Neuman"@Mad-Host,Sam.Irving@Other-Host
Message-ID:  <some.string@SHOST>

A.3.3. About as complex as you're going to get

Date     :  27 Aug 76 0932 PDT
From     :  Ken Davis <KDavis@This-Host.This-net>
Subject  :  Re: The Syntax in the RFC
Sender   :  KSecy@Other-Host
Reply-To :  Sam.Irving@Reg.Organization
To       :  George Jones <Group@Some-Reg.An-Org>,Al.Neuman@MAD.Publisher
cc       :  Important folk:Tom Softwood <Balsa@Tree.Root>,"Sam Irving"@Other-Host;,Standard Distribution:/main/davis/people/standard@Other-Host,"<Jones>standard.dist.3"@Tops-20-Host>;
Comment  :  Sam is away on business. He asked me to handlehis mail for him.  He'll be able to provide  amore  accurate  explanation  when  he  returnsnext week.
In-Reply-To: <some.string@DBM.Group>, George's message
X-Special-action:  This is a sample of user-defined field-names.  There could also be a field-name"Special-action", but its name might later bepreempted
Message-ID: <4231.629.XYzi-What@Other-Host>

Appendix B. Simple Field Parsing

Some mail-reading software systems may wish to perform only minimal processing, ignoring the internal syntax of structured field-bodies and treating them the same as unstructured-field-bodies. Such software will need only to distinguish:

• Header fields from the message body,
• Beginnings of fields from lines which continue fields,
• Field-names from field-contents.

The abbreviated set of syntactic rules which follows will suffice for this purpose. It describes a limited view of messages and is a subset of the syntactic rules provided in the main part of this specification. One small exception is that the contents of field-bodies consist only of text:

B.1. SYNTAX

message         =   *field *(CRLF *text)field           =    field-name ":" [field-body] CRLFfield-name      =  1*<any CHAR, excluding CTLs, SPACE, and ":">field-body      =   *text [CRLF LWSP-char field-body]

B.2. SEMANTICS

Headers occur before the message body and are terminated  by
a null line (i.e., two contiguous CRLFs).A line which continues a header field begins with a SPACE or
HTAB  character,  while  a  line  beginning a field starts with a
printable character which is not a colon.A field-name consists of one or  more  printable  characters
(excluding  colon,  space, and control-characters).  A field-name
MUST be contained on one line.  Upper and lower case are not dis-
tinguished when comparing field-names.

Apppendix C. DIFFERENCES FROM RFC #733

The following summarizes the differences between this  stan-
dard  and the one specified in Arpanet Request for Comments #733,
"Standard for the Format of ARPA  Network  Text  Messages".   The
differences  are  listed  in the order of their occurrence in the
current specification.C.1.  FIELD DEFINITIONSC.1.1.  FIELD NAMESThese now must be a sequence of  printable  characters.   Theymay not contain any LWSP-chars.C.2.  LEXICAL TOKENSC.2.1.  SPECIALSThe characters period ("."), left-square  bracket  ("["),  andright-square  bracket ("]") have been added.  For presentationpurposes, and when passing a specification to  a  system  thatdoes  not conform to this standard, periods are to be contigu-ous with their surrounding lexical tokens.   No  linear-white-space  is  permitted  between them.  The presence of one LWSP-char between other tokens is still directed.C.2.2.  ATOMAtoms may not contain SPACE.C.2.3.  SPECIAL TEXTctext and qtext have had backslash ("\") added to the list  ofprohibited characters.C.2.4.  DOMAINSThe lexical tokens  <domain-literal>  and  <dtext>  have  beenadded.C.3.  MESSAGE SPECIFICATIONC.3.1.  TRACEThe "Return-path:" and "Received:" fields have been specified.C.3.2.  FROMThe "From" field must contain machine-usable addresses  (addr-spec).   Multiple  addresses may be specified, but named-lists(groups) may not.C.3.3.  RESENTThe meta-construct of prefacing field names  with  the  string"Resent-"  has been added, to indicate that a message has beenforwarded by an intermediate recipient.C.3.4.  DESTINATIONA message must contain at least one destination address field."To" and "CC" are required to contain at least one address.C.3.5.  IN-REPLY-TOThe field-body is no longer a comma-separated list, although asequence is still permitted.C.3.6.  REFERENCEThe field-body is no longer a comma-separated list, although asequence is still permitted.C.3.7.  ENCRYPTEDA field has been specified that permits  senders  to  indicatethat the body of a message has been encrypted.C.3.8.  EXTENSION-FIELDExtension fields are prohibited from beginning with the  char-acters "X-".

C.4. DATE AND TIME SPECIFICATION

C.4.1.  SIMPLIFICATIONFewer optional forms are permitted  and  the  list  of  three-letter time zones has been shortened.

C.5. ADDRESS SPECIFICATION

C.5.1. ADDRESS

The use of quoted-string, and the ":"-atom-":" construct, havebeen  removed.   An  address  now  is  either a single mailboxreference or is a named list of addresses.  The  latter  indi-cates a group distribution.C.5.2.  GROUPSGroup lists are now required to to have a name.   Group  listsmay not be nested.

C.5.3. MAILBOX

A mailbox specification  may  indicate  a  person's  name,  asbefore.   Such  a  named  list  no longer may specify multiplemailboxes and may not be nested.

C.5.4. ROUTE ADDRESSING

Addresses now are taken to be absolute, global specifications,independent  of transmission paths.  The <route> construct hasbeen provided, to permit explicit specification  of  transmis-sion  path.   RFC  #733's  use  of multiple at-signs ("@") wasintended as a general syntax  for  indicating  routing  and/orhierarchical addressing.  The current standard separates thesespecifications and only one at-sign is permitted.

C.5.5. AT-SIGN

The string " at " no longer is used as an address delimiter. Only at-sign ("@") serves the function.

C.5.6. DOMAINS

Hierarchical, logical name-domains have been added.

C.6. RESERVED ADDRESS

The local-part "Postmaster" has been reserved, so that users can be guaranteed at least one valid address at a site.

Appendix D. Alphabetical Listing of Syntax Rules

address     =  mailbox                      ; one addressee/  group                        ; named list
addr-spec   =  local-part "@" domain        ; global address
ALPHA       =  <any ASCII alphabetic character>; (101-132, 65.- 90.); (141-172, 97.-122.)
atom        =  1*<any CHAR except specials, SPACE and CTLs>
authentic   =   "From"       ":"   mailbox  ; Single author/ ( "Sender"     ":"   mailbox  ; Actual submittor"From"       ":" 1#mailbox) ; Multiple authors;  or not sender
CHAR        =  <any ASCII character>        ; (  0-177,  0.-127.)
comment     =  "(" *(ctext / quoted-pair / comment) ")"
CR          =  <ASCII CR, carriage return>  ; (     15,      13.)
CRLF        =  CR LF
ctext       =  <any CHAR excluding "(",     ; => may be folded")", "\" & CR, & includinglinear-white-space>
CTL         =  <any ASCII control           ; (  0- 37,  0.- 31.)character and DEL>          ; (    177,     127.)
date        =  1*2DIGIT month 2DIGIT        ; day month year;  e.g. 20 Jun 82
dates       =   orig-date                   ; Original[ resent-date ]               ; Forwarded
date-time   =  [ day "," ] date time        ; dd mm yy;  hh:mm:ss zzz
day         =  "Mon"  / "Tue" /  "Wed"  / "Thu"/  "Fri"  / "Sat" /  "Sun"
delimiters  =  specials / linear-white-space / comment
destination =  "To"          ":" 1#address  ; Primary/  "Resent-To"   ":" 1#address/  "cc"          ":" 1#address  ; Secondary/  "Resent-cc"   ":" 1#address/  "bcc"         ":"  #address  ; Blind carbon/  "Resent-bcc"  ":"  #address
DIGIT       =  <any ASCII decimal digit>    ; ( 60- 71, 48.- 57.)
domain      =  sub-domain *("." sub-domain)
domain-literal =  "[" *(dtext / quoted-pair) "]"
domain-ref  =  atom                         ; symbolic reference
dtext       =  <any CHAR excluding "[",     ; => may be folded"]", "\" & CR, & includinglinear-white-space>
extension-field =<Any field which is defined in a documentpublished as a formal extension to thisspecification; none will have names beginningwith the string "X-">field       =  field-name ":" [ field-body ] CRLF
fields      =    dates                      ; Creation time,source                     ;  author id & one1*destination                ;  address required*optional-field             ;  others optional
field-body  =  field-body-contents[CRLF LWSP-char field-body]
field-body-contents =<the ASCII characters making up the field-body, asdefined in the following sections, and consistingof combinations of atom, quoted-string, andspecials tokens, or else consisting of texts>
field-name  =  1*<any CHAR, excluding CTLs, SPACE, and ":">
group       =  phrase ":" [#mailbox] ";"
hour        =  2DIGIT ":" 2DIGIT [":" 2DIGIT]; 00:00:00 - 23:59:59
HTAB        =  <ASCII HT, horizontal-tab>   ; (     11,       9.)
LF          =  <ASCII LF, linefeed>         ; (     12,      10.)
linear-white-space =  1*([CRLF] LWSP-char)  ; semantics = SPACE; CRLF => folding
local-part  =  word *("." word)             ; uninterpreted; case-preserved
LWSP-char   =  SPACE / HTAB                 ; semantics = SPACE
mailbox     =  addr-spec                    ; simple address/  phrase route-addr            ; name & addr-spec
message     =  fields *( CRLF *text )       ; Everything after;  first null line;  is message body
month       =  "Jan"  /  "Feb" /  "Mar"  /  "Apr"/  "May"  /  "Jun" /  "Jul"  /  "Aug"/  "Sep"  /  "Oct" /  "Nov"  /  "Dec"
msg-id      =  "<" addr-spec ">"            ; Unique message id
optional-field =/  "Message-ID"        ":"   msg-id/  "Resent-Message-ID" ":"   msg-id/  "In-Reply-To"       ":"  *(phrase / msg-id)/  "References"        ":"  *(phrase / msg-id)/  "Keywords"          ":"  #phrase/  "Subject"           ":"  *text/  "Comments"          ":"  *text/  "Encrypted"         ":" 1#2word/  extension-field              ; To be defined/  user-defined-field           ; May be pre-empted
orig-date   =  "Date"        ":"   date-time
originator  =   authentic                   ; authenticated addr[ "Reply-To"   ":" 1#address] )
phrase      =  1*word                       ; Sequence of wordsqtext       =  <any CHAR excepting <">,     ; => may be folded"\" & CR, and includinglinear-white-space>
quoted-pair =  "\" CHAR                     ; may quote any char
quoted-string = <"> *(qtext/quoted-pair) <">; Regular qtext or;   quoted chars.
received    =  "Received"    ":"            ; one per relay["from" domain]           ; sending host["by"   domain]           ; receiving host["via"  atom]             ; physical path*("with" atom)             ; link/mail protocol["id"   msg-id]           ; receiver msg id["for"  addr-spec]        ; initial form";"    date-time         ; time receivedresent      =   resent-authentic[ "Resent-Reply-To"  ":" 1#address] )
resent-authentic ==   "Resent-From"      ":"   mailbox/ ( "Resent-Sender"    ":"   mailbox"Resent-From"      ":" 1#mailbox  )
resent-date =  "Resent-Date" ":"   date-time
return      =  "Return-path" ":" route-addr ; return address
route       =  1#("@" domain) ":"           ; path-relative
route-addr  =  "<" [route] addr-spec ">"
source      = [  trace ]                    ; net traversalsoriginator                 ; original mail[  resent ]                   ; forwarded
SPACE       =  <ASCII SP, space>            ; (     40,      32.)
specials    =  "(" / ")" / "<" / ">" / "@"  ; Must be in quoted-/  "," / ";" / ":" / "\" / <">  ;  string, to use/  "." / "[" / "]"              ;  within a word.
sub-domain  =  domain-ref / domain-literal
text        =  <any CHAR, including bare    ; => atoms, specials,CR & bare LF, but NOT       ;  comments andincluding CRLF>             ;  quoted-strings are;  NOT recognized.
time        =  hour zone                    ; ANSI and Military
trace       =    return                     ; path to sender1*received                   ; receipt tags
user-defined-field =<Any field which has not been definedin this specification or published as anextension to this specification; names forsuch fields must be unique and may bepre-empted by published extensions>
word        =  atom / quoted-stringzone        =  "UT"  / "GMT"                ; Universal Time; North American : UT/  "EST" / "EDT"                ;  Eastern:  - 5/ - 4/  "CST" / "CDT"                ;  Central:  - 6/ - 5/  "MST" / "MDT"                ;  Mountain: - 7/ - 6/  "PST" / "PDT"                ;  Pacific:  - 8/ - 7/  1ALPHA                       ; Military: Z = UT;
<">         =  <ASCII quote mark>           ; (     42,      34.)