Makes an indexed mail file smaller by recovering space that is
    left by deleted mail messages. When you compress a file, the
    following steps occur:

    1. A temporary file named MAIL_nnnn_COMPRESS.TMP is created.
       (nnnn is a hexadecimal, 4- or 8-digit number.)

    2. The contents of the file to be compressed are copied to the
       temporary file, and the temporary file is then compressed.

    3. The original uncompressed file is renamed with a file type
       .OLD.

    4. The newly compressed file is renamed from
       MAIL_nnnn_COMPRESS.TMP back to its original name.

    Format

      COMPRESS  [file-spec]