Djigzo Email Encryption Gateway is a standards based centrally managed email server (MTA) that encrypts and decrypts your incoming and outgoing email at the gateway level. Because Djigzo Email Encryption Gateway functions as a general SMTP email server, it is compatible with any existing email infrastructure.


http://www.djigzo.com/

http://www.djigzo.com/documents/faq.pdf - good for beginners in PKI.