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

    OpenVMS usage:cond_value
    type:         longword (unsigned)
    access:       write only
    mechanism:    by value

    Longword condition value. All utility routines return a condition
    value in R0. Condition values that can be returned by this
    routine are listed under Condition Values Returned.