rfc724.txt

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            These fields will be referred  to  as  structured  fields.
            Examples  include  fields  containing dates and addresses.
            Other fields, such as  the  subject  field,  are  regarded
            simply as a single line of text.

         3) Field names

            To aid in the creation and reading of  <field-name>s,  the
            free   insertion  of  <linear-white-space>  characters  is
            allowed in reasonable places.  Rather than  obscuring  the
            syntax  specification  for  <field-name> with the explicit
            syntax  for  these  <linear-white-space>  characters,  the
            existence  of a simple "lexical" analyzer is assumed.  The
            analyzer reinterprets the unfolded  text  which  comprises
            the  <field-name>  as  a  sequence of <atoms> separated by
            <linear-white-space> characters.  The field  name  may  be
            conveniently  represented  by the sequence of these atoms,
            separated by a single ASCII space character.



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         4) Field bodies

            To aid in the creation and reading  of  structured fields,
            the  free  insertion of <linear-white-space> characters is
            allowed in reasonable places.  Rather than  obscuring  the
            syntax specifications for  these  structured  fields  with
            explicit syntax for these <linear-white-space> characters,
            the existence of  another  simple  "lexical"  analyzer  is
            assumed.   It  provides  an interpretation of the unfolded
            text comprising the body of the field  as  a  sequence  of
            lexical symbols.  These include

                    -  individual special characters
                    -  quoted strings
                    -  comments
                    -  atoms

            The first three symbols are  self-delimiting.   Atoms  are
            not;  they  therefore are delimited by the self-delimiting
            symbols and by <linear-white-space>.

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

                    ":sysmail"@ Some-Host,
                    Muhammed(I am the greatest)Ali at WBA

            is analyzed into the following lexical symbols and types:

                    ":sysmail"              quoted string
                    @                       special
                    Some-Host               atom
                    ,                       special
                    Muhammed                atom
                    (I am the greatest)     comment
                    Ali                     atom
                    at                      atom
                    WBA                     atom


     b.  Formal Definition

         <field>           ::=   <field-name> ":" <field-body>
         <field-name>      ::=   <atom>
                               | <atom> <field-name>

         <field-body>      ::=   <field-body-contents>
                               | <field-body-contents> <crlf>
                                    <linear-white-space-char>
                                    <field-body>



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         <field-body-contents> ::= <the TELNET ASCII characters making
                                    up the <field-body>, as defined in
                                    the following sections, and
                                    consisting of combinations of
                                    <atom>, <quoted-string>, <text-line>,
                                    and <specials> tokens>

         <atom>            ::=   <a sequence of one or more TELNET
                                    ASCII alpha-numeric or graphics
                                    characters, excluding all control
                                    characters (those characters with
                                    a decimal value less than 33 or
                                    equal to 127) and <delimeters> >

         <quoted-string>   ::=   <double quote mark ("), decimal 34>
                                    <a sequence of one or more TELNET
                                    ASCII characters, where two
                                    adjacent quotes are treated as a
                                    single quote and part of the
                                    string> <">

         <text-line>        ::=   <a sequence of one or more TELNET
                                    ASCII characters excluding <cr>
                                    and <lf> >

         <message-text>     ::=   <a sequence of zero of more TELNET
                                    ASCII characters>

         <delimeters>      ::=   <specials> | <comment>
                               | <linear-white-space> | <crlf>

         <specials>        ::=   "(" | ")" | "<" | ">"
                               | "@" | "," | ";" | ":" | <">

         <comment>         ::=   "(" <TELNET ASCII characters, except
                                    <crlf> > ")"

         <linear-white-space>::= <linear-white-space-char>
                               | <linear-white-space-char>
                                    <linear-white-space>
         <linear-white-space-char>::=  <space> | <horizontal-tab>

         <space>           ::=   <TELNET ASCII space (decimal 32)>
         <tab>             ::=   <TELNET ASCII tab   (decimal  9)>
         <cr>              ::=   <TELNET ASCII carriage return
                                    (decimal 13)>
         <lf>              ::=   <TELNET ASCII line feed (decimal 10)>
         <crlf>            ::=   <TELNET ASCII carriage return/line
                                  feed (decimal 13, followed by
                                  decimal 10)>



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     c.  Clarifications

         1) Comments

            Comments  may  appear   only   within   <field-body>s   of
            structured fields.   A  comment is any set of TELNET ASCII
            characters, which is not within a quoted string, and which
            is  enclosed in matching parentheses; parentheses nest, so
            that if a left paren occurs in  a  comment  string,  there
            must also be a matching right paren.

            Comments are NOT passed to the FTP server, as  part  of  a
            MAIL  or  MLFL command, since comments are not part of the
            "formal" address.


         2) "White space"

            Remember that in structured fields, MULTIPLE LINEAR  WHITE
            SPACE TELNET ASCII CHARACTERS (namely <tab>s and <space>s)
            ARE TREATED AS SINGLE SPACES AND MAY FREELY  SURROUND  ANY
            SYMBOL.   In  all  header  fields, at least one <space> is
            REQUIRED only at the beginning of folded lines.

            Writers of mail-sending (i.e.  header generating) programs
            should realize that there is no Network-wide definition of
            the  effect  of  <tab>  TELNET  ASCII  characters  on  the
            appearance of text at another Network host; therefore, the
            use of <tab>s in message  headers,  though  permitted,  is
            discouraged.

            Note that the contents of messages are required to conform
            with  TELNET  NVT conventions (e.g.  <cr> must be followed
            by either <lf>, making a <crlf>, or <null>, if the <cr> is
            to stand alone).

         3) Quoted strings

            Where  permitted  (i.e.,  in  structured  fields)   quoted
            strings  are  treated as a single symbol (i.e.  equivalent
            to an <atom> syntactically).  However, if  quoted  strings
            are  to  be  "folded" onto multiple lines, then the syntax
            for folding must be  adhered  to  (See  items  II.B.1.a.1,
            above,  and  II.B.1.c.6,  below.)  Note  that the official
            semantics do not  encounter  <crlf>s  in  quoted  strings,
            although  particular  parsing  programs  may  wish to note
            their presence.



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      1. Lexical Analysis



         4) Bracketing characters

            There are two types of brackets which must be well nested:

                - Parentheses are used to indicate comments.

                - Angle brackets  ("<"  and  ">")  are  used
                  where  there is a question of the presence
                  of machine-usable code (e.g.  deliminating
                  mailboxes).

         5) Case independence of certain specials <atom>s

            It should be assumed by all  mail  reading  programs  that
            certain  <atom>s  can be represented in any combination of
            upper and lower case.  These are:

                - <field-name>s,
                - "File", in a <path>,
                - "at", in an <at-indicator>,
                - <host-name>s,
                - <day-of-week>s,
                - <string-month>s, and
                - <time-zone>s

            For example, the <field-name>s "From", "FROM", "from", and
            even "FroM" should all be treated identically.  Note that,
            at the level of this specification, case  IS  relevant  to
            other   <word>s   and   <text-line>s.   Also  see  Section
            II.C.1.a.4, below.

         6) Folding long lines

            Each header item (field of the message) may be represented
            on  exactly  one  line consisting of the name of the field
            and its body, and this  is  what  the  parser  sees.   For
            readability,  it  is  recommended  that  the  <field-body>
            portion of long header items  be  "folded"  onto  multiple
            lines of the actual header.


         7) Backspace characters

            Backspace TELNET ASCII characters (ASCII  BS,  decimal  8)
            may  be  included  in  <text-line>  and <quoted-string> to
            effect overstriking; however, any use of backspaces  which
            effects  an overstrike to the left of the beginning of the
            <text-line> or <quoted-string> is prohibited.




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      2. Messages



     2.  GENERAL SYNTAX OF MESSAGES:

         NOTE: The syntax indicates that items  in  <required-headers>
         must  be  in  a  specific  order and precede all other header
         items.  Header fields, in fact, are NOT required to occur  in
         any  particular  order.  Required header items must be unique
         (occur exactly once).  This  specification  permits  multiple
         occurrences   of   most   optional   fields.    However,  the
         interpretation of such multiple occurrences is not  specified
         here.

         <message>         ::=   <headers>
                               | <headers> <crlf> <message-text>

         <headers>         ::=   <required-headers>
                               | <required-headers> <optional-headers>
         <required-headers> ::=  <date-field> <originator>
         <originator>      ::=   <mach-from-field>
                               | <mach-from-list> <sender-field>
                               | <mach-from-field> <reply-to-field>
                               | <any-from-field> <sender-field>
                                    <reply-to-field>

         <date-field>      ::=   "Date"        ":" <date-time>
         <mach-from-field> ::=   "From"        ":" <mach-addr-item>
         <mach-from-list>  ::=   "From"        ":" <mach-addr-list>
         <any-from-field>  ::=   "From"        ":" <address-list>
         <sender-field>    ::=   "Sender"      ":" <host-phrase>
         <reply-to-field>  ::=   "Reply-To"    ":" <mach-addr-list>

         <optional-headers>::=   <optional-header-field>
                               | <optional-headers>
                                    <optional-header-field>

         <optional-header-field> ::= <addressee-field>
                               | <extension-field>

         <addressee-field> ::=   "To"          ":" <address-list>
                               | "cc"          ":" <address-list>
                               | "bcc"         ":" <address-list>
                               | "Fcc"         ":" <path-list>

         <extension-field> ::=   "In-Reply-To" ":" <reference-list>
                               | "Keywords"    ":" <phrase-list>
                               | "Message-Id"  ":" <mach-host-phrase>
                               | "References"  ":" <reference-list>
                               | "Subject"     ":" <text-line>
                               | "Comments"    ":" <text-line>
                               | <user-defined-field>



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      2. Messages



         <user-defined-field> ::= <A <field> which has a <field-name>
                                    not defined in this specification>



     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 section  on  Lexical  Analysis  (section  II.B.1)