The mailtool of SUNs OpenWindows lacks numerous Things, including the
Ability to sign Mails. However, most OW Users stick with mailtool
because of the Ability to use "Attachments" to send around Files.

A simple Replacement for Signatures is to add a "Template" (click on
Edit -> Properties, select Category "Template" in the Properties
Window, and give Name and File as desired). Disadvantage: You have
to edit in every Signature by Hand, though.

A better Approach is to use a "set sendmail=..." Line in your ~/.mailrc.
Mails being sent out will be handed over to the Executable named
there instead of the Mail Delivery Subsystem. You can easily plug
in a simple Program to sign your Mails there. However, be warned that
all too simple Siggers aren't aware of the abovementioned Attachments,
so the Signature will end up in the last Attachment instead of the
Mail Text. Information about a Sigger that handles mail containing
attachments correctly can be obtained from Jochen Berg by sending
email to: mailtool-sig@ti.uni-trier.de