VisNetic MailServer

From Deerfield.com:

VisNetic MailServer is a high performance POP3, SMTP, IMAP, LDAP email server for Windows operating systems. It features a powerful, comprehensive antispam engine that uses three proven spam filtering technologies to ensure the highest possible level of spam protection. Integrated antivirus support, a built-in Instant Messaging server, Web Mail and support for VisNetic GroupWare combine to make it a superior messaging solution for customers reliant upon email for business communications.
VisNetic MailServer includes Instant AntiSpam, a powerful Anti Spam tool that employs: A Bayesian filter that statistically calculates the probability that each inbound message is spam; The SpamAssassin rules-set, recognized as the most exhaustive list of spam filters to date; A Challenge-Response system that ensures that messages are being sent from a human being, as opposed to machines. In addition to these anti spam tools, VisNetic MailServer also features standard relay checking, reverse DNS lookup, MX record verification, RBL checking and extensive email content filtering.
VisNetic MailServer features integrated and secure Instant Messaging (IM) for concise, immediate communication between employees at one or more locations in your organization and with partners and customers all over the world. This secure, proprietary IM server is compatible with the Jabber open XML protocol and instant messages between the IM server and any Jabber-compatible IM client are secured using SSL/TLS encryption. VisNetic MailServer is a multithreaded application, featuring multiple CPU support and an innovative High Speed Memory Mode. It supports ODBC connections, multiple domains and POP3, SMTP, IMAP, LDAP, HTTP and WAP email protocols. VMS includes support for secure TLS/SSL connections, encrypting email messages between servers and servers and clients, and remote administration options, giving administrators complete control of the mail server through a secure browser connection.