rspamd (An advanced spam filtering system)

Rspamd is an advanced spam filtering system 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 spam score.

According to this spam score and the user's settings Rspamd recommends
an action for the MTA to apply to the message: for example, to pass, to
reject or to add a header. Rspamd is designed to process hundreds of
messages per second simultaneously and has a number of features
available.