Mail transport systems are slow to adopt new technology, and spammers are quick and flexible, able to adopt new technology as quickly as it becomes available. Consequently, most SMTP servers are ill equipped to stop spam.
Furthermore, one spam-stopping solution could work with all existing SMTP servers if it was implemented on a second level -- a transparent SMTP filtering proxy. This was my goal for this project.
wanted a server that accepted connections on port 25, passing the transmission on to the official SMTP server, and relaying its replies back to the SMTP client. But when enough of the message had been transmitted to validate its legitimacy the ASSP server could either pipe the remainder of the message to the official SMTP server or close the connection to the official SMTP server and ignore the remainder of the message.
The Anti-Spam SMTP Proxy (ASSP) Server project is an open source platform-independent SMTP Proxy server which implements whitelists and Bayesian filtering to rid the planet of the blight of unsolicited email (UCE). UCE must be stopped at the SMTP server. Anti-spam tools must be adaptive to new spam and customized for each site's mail patterns. This free, easy-to-use tool works with any mail transport and achieves these goals requiring no operator intervention after the initial setup phase.
