Rspamd is an advanced spam filtering system and email processing
framework that allows evaluation of messages by a number of rules
including regular expressions, statistical analysis and custom services
such as URL black lists. Each message is analysed by Rspamd and given a
verdict that might be used by MTA for further processing (e.g. to reject
a message, or add a special header indicating spam) along with other
information, such as possible DKIM signature or modifications suggested
for a message.

Rspamd is designed to process hundreds of messages per second
simultaneously, and provides a number of useful features including a
comprehensive Lua API that allows access to messages processing in
various aspects as well as asynchronous network API to access external
resources, such as DNS, HTTP or even generic TCP/UDP services.

Homepage:
https://www.rspamd.com/
