VMS Help MAIL Routines, MAIL$SEND_ADD_ATTRIBUTE, Description  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    If you do not specify a To: line, the Mail utility supplies a
    To: line composed of user names on the To: address list. If you
    do not specify a CC: line, the Mail utility supplies a CC: line
    composed of user names on the CC: address list. In either of the
    above cases, commas separate the user names.

    To add a message's From: field, you must have the SYSPRV
    privilege, and the Mail DECnet object must have the SYSPRV
    privilege on OUTGOING CONNECT (users can set the DECnet object
    privileges at their discretion).