gpg-mailgate is a small content filter for Postfix + web interface that allows any user to upload PGP keys to your server (and confirm email address), and then will begin encrypting any outgoing mail ...
I have setup postfix on my openbsd mail server, and made use of the header_checks and body_checks and main.cf retrictions from securitysage.com, in an attempt to lower the amount of spam I receive ...