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

    If you do not specify a MAIL$_SEND_USERNAME_TYPE, MAIL$SEND_ADD_
    ADDRESS uses MAIL$_TO. You can specify only one user name per
    call to MAIL$SEND_ADD_ADDRESS.