Fast, free and open-source spam filtering system
https://rspamd.com/

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, reject or add
a header.  Rspamd is designed to process hundreds of messages per second
simultaneously and has a number of features available.