From Server Side Solutions:
eWall is a Microsoft Windows based gateway between the Internet and your mail server. It monitors the SMTP traffic in real-time, catching the majority of spam and viruses. The chain of flexible filters stops more than 95% of unwanted e-mails out of the box (more than 99% with Message Sniffer), dropping the mail server load by more than 90%. eWall doesn't require maintenance and includes optional self-updating components (ClamAV, Message Sniffer), that seamlessly work with and protect your mail environment from spam and viruses 24 hours a day, 7 days a week. eWall is built on top of time-proven and reliable technologies. PHP is used as a flexible and powerful scripting engine, allowing you to write filters of any complexity. Any standard PHP drivers or libraries can be used in eWall's event-driven environment. The powerful PostgreSQL database is used to keep the statistics and index quarantined messages, e-mails and IP-lists, and track the session history. XML-RPC transport is used to manage and watch the eWall remotely from the Control Panel.
All filters represent open-sourced snippets of PHP code, stored in the eWall Repository. The Repository is an XML-formatted file with well documented structure, allowing you to easily modify existing, or quicky develop new, filters, containing the newest anti-spam and anti-virus techniques. The Repository shipped with eWall already contains most of major anti-spam and anti-virus filters, such as DNSBL, SPF, SURBL, Attachment filtering, Country filtering, SpamAssassin, ClamAV and various misc tests. eWall is available in two editions: Free and Unlimited. The Free edition is limited to the number of mailboxes and intended for small companies. The Unlimited edition protects an unlimited number of mailboxes and is available on a subscription basis. At any time you can upgrade from the Free edition to the Unlimited edition without re-installation, by replacing only the license key file.