Library /sys$common/syshlp/mailhelp.hlb SET-SHOW, SIGNATURE_FILE, Parameter  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 file-name

    Specifies the name of the default signature file (text) that will
    be appended to each mail message.

    When specifying the signature file name, note the following:

    o  If you do not specify a file type, the default is .SIG.

    o  If you do not specify a directory, the Mail utility will
       search for the signature file in your mail directory.