Rspamd

Rspamd is a spam filtering system and email processing framework. It evaluates each message against many rules, including regular expressions, statistical analysis and custom services such as URL blacklists, and returns a verdict that a mail transfer agent can act on, for example by rejecting the message or adding a header that marks it as spam.

It can act as a Milter, which lets it work directly with common mail servers such as Postfix and Sendmail. The results can also include DKIM signature details and suggested modifications to a message. Rspamd is designed to process many messages in parallel and offers a comprehensive Lua API, plus an asynchronous network API for reaching external resources like DNS, HTTP and generic TCP or UDP services.

Many spam filtering modules and features are enabled out of the box, and you can write your own rules and plugins in Lua. It is written in C and packaged for the major Linux distributions as well as FreeBSD ports, NetBSD pkgsrc and OpenBSD ports. The project recommends its own packages over distribution ones, which may be outdated. A quick start guide explains installation and configuration.