/  "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 id






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     Standard for ARPA Internet Text Messages


     extension-field =
                   <Any field which is defined in a document
                    published as a formal extension to this
                    specification; none will have names beginning
                    with the string "X-">

     user-defined-field =
                   <Any field which has not been defined
                    in this specification or published as an
                    extension to this specification; names for
                    such fields must be unique and may be
                    pre-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 the
            set of original fields.  Information for  one  set  should
            not  automatically be taken from the other.  The interpre-
            tation of multiple "Resent-" fields, of the same type,  is
            undefined.

          In the remainder of this specification, occurrence of  legal
     "Resent-"  fields  are treated identically with the occurrence of








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     Standard for ARPA Internet Text Messages


     fields whose names do not contain this prefix.

     4.3.  TRACE FIELDS

          Trace information is used to provide an audit trail of  mes-
     sage  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  that
        delivers  the message to its recipient.  The field is intended
        to contain definitive information about the address and  route
        back to the message's originator.