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

    The MAIL$USER_SET_INFO routine modifies specified records in
    the user profile database. If you do not specify a user name,
    the routine modifies the user record associated with the calling
    process.

    To modify any user record other than that associated with the
    calling process, you must have SYSPRV privilege. However, if
    you want to add or modify only the forwarding address of another
    user, SYSNAM privilege is sufficient.