Library /sys$common/syshlp/mailhelp.hlb COMPRESS, Qualifier, /OUTPUT  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /OUTPUT=out-file-spec

    The name of the compressed file. Note that if you specify an
    output file, the mail file is compressed and renamed. This leaves
    you with 2 copies of your mail file: the original uncompressed
    version and the new compressed version that has the name that you
    specified when you entered the /OUTPUT qualifier.