Rspamd Fast, free and open-source spam filtering system.
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 can act as a Milter allowing direct interaction with popular MTA systems, such as Postfix or Sendmail.
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.