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

    If you do not specify an input file, the MAIL$MAILFILE_
    COMPRESS routine compresses the currently open Mail file. The
    MAIL$MAILFILE_COMPRESS routine signals informational messages
    concerning the phase of the compression.