The Anti-Spam SMTP Proxy (ASSP) Server project aims to create an open source platform-independent SMTP Proxy server which implements auto-whitelists, self learning Bayesian, Greylisting, DNSBL, DNSWL, URIBL, SPF, SRS, Backscatter, Virus scanning, attachment blocking, Senderbase and multiple other filter methods. Click ‘Browse all files’ to download the professional version 2.2.1 build 12313.

Features:

Auto Update
Control Outgoing
Unique Backscatter Detection and Blocking
Foreign Character Support for Regex Filter
Native SSL-support. ASSP will be “Man in the Middle” and will do TLS/SSL-connections even if your MTA don’t support it!
Resending of blocked eMail to the original recipients by clicking a link from the new eMail Block reports or from the maillog tail
Improved HELO handling
Classes of DNSBL lists
Weighting of Regular Expression
Improved community Griplist (GreyIP List)
Improved performance and memory usage
Additional to ClamAV you can now use your filesystem virusscanner for scanning your eMails
Lots of more usefull tweaks and setting

Download free ASSP here