The open source Anti-Spam SMTP Proxy Server project was designed to create a platform-independent SMTP Proxy server that implements auto-whitelists, self learning Bayesian, Greylisting, DNSBL, DNSWL, URIBL, SPF, SRS, Backscatter, Virus scanning, attachment blocking, SenderBase and multiple other filter methods.
Key features include:
Anti-Spam SMTP Proxy is a spam filter that sits on port 25 (SMTP), and if required, also on 465 (SMTPS) and 587 (Submission), directly in front of your regular SMTP server. It will relay the SMTP dialog between an incoming client and your SMTP server. It will intercept the dialog as it is required. Anti-Spam SMTP Proxy performs a number of configurable checks and upon detecting spam messages it will provide an immediate 5xx SMTP error code to the client. As usual, any non-spam messages are passed to the regular SMTP server for further processing and delivery.
Anti-Spam SMTP Proxy Server has numerous configurable features, such as the ability to block or subject tag and deliver spam messages. On the downside, if you are unfamiliar with this kind of application, it can be hard to setup and maintain.