The CommuniGate Pro Secure Mail (S/MIME) functionality is based on the Public Key technology. Using S/MIME, you can:

• digitally sign your message, so the recipient can:
  • verify that the message has been really sent by you;
  • verify that the message content has not been altered and that it was received in exactly the same form as it was composed by you (the sender);
• digitally encrypt your message, so only the recipients can read it, even if the message has been intercepted while it was being transferred, or if it has been copied from the server files that store the message.

http://www.stalker.com/cgatepro/WebSMIME.html