ssmail was a patch to Sendmail which enables opportunistic encryption of mail transfer sessions. This was developed by Damian Bentley while he worked as a summer intern for QUALCOMM Australia.

A more detailed paper about ssmail was presented at the 1999 USENIX LISA conference. Here is ssmail_LISA.pdf.

What ssmail was intended to achieve is now done in a more standard way using the "STARTTLS" SMTP option. As far as I know, no-one currently runs ssmail, so there isn't any point you running it either. For that reason, we've deleted the downloadable code. But I'm pleased that it helped to stimulate other development.