Note:  The "Reply-To" field is added  by  the  originator  and
               serves  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  is
        optional,  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  that
        relays the message.  The information in the field can be quite
        useful 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, to
        indicate what physical mechanism the message  was  sent  over,
        such  as  Arpanet or Phonenet, and the "with" parameter may be
        used to indicate the mail-,  or  connection-,  level  protocol
        that  was  used, such as the SMTP mail protocol, or X.25 tran-
        sport protocol.

        Note:  Several "with" parameters may  be  included,  to  fully
               specify 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  destination
        address specification that the receiving host reinterprets, by


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        expansion or transformation, the receiving host  may  wish  to
        record  the original specification, using the "for" parameter.
        For example, when a copy of mail is sent to the  member  of  a
        distribution  list,  this  parameter may be used to record the
        original 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  wished
        this  message to be sent.  The message-creation process should
        default this field  to  be  a  single,  authenticated  machine
        address,  indicating  the  AGENT  (person,  system or process)
        entering the message.  If this is not done, the "Sender" field
        MUST  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" field
        must be machine-usable (addr-specs) and may not contain  named
        lists (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 is
        intended for use when the sender is not the author of the mes-
        sage,  or  to  indicate  who among a group of authors actually
        sent the message.  If the contents of the "Sender" field would
        be  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 MUST
        be present if it is NOT the same as the "From" Field.

        The Sender mailbox  specification  includes  a  word  sequence
        which  must correspond to a specific agent (i.e., a human user
        or a computer program) rather than a standard  address.   This
        indicates  the  expectation  that  the field will identify the
        single AGENT (person,  system,  or  process)  responsible  for
        sending  the mail and not simply include the name of a mailbox
        from which the mail was sent.  For example in the  case  of  a
        shared login name, the name, by itself, would not be adequate.
        The local-part address unit, which refers to  this  agent,  is
        expected to be a computer system term, and not (for example) a
        generalized person reference which can  be  used  outside  the
        network text message context.


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        Since the critical function served by the  "Sender"  field  is
        identification  of  the agent responsible for sending mail and