Fast, free and open-source spam filtering system.
Rspamd is optimised to work as fast as possible. There are numerous optimisations that allow mail to be processed faster. Rspamd uses an event based processing model and does not block anywhere in the code. Moreover, it is possible to use multiple threads for parallelising complex tasks and multiple processes to serve requests. This allows the processing of hundreds of messages per second without significant CPU load.
Rspamd supports various internal and external filters to process messages, such as regular expressions, suffix tries, RBLs, URL black lists, IP lists, SPF, DKIM, character maps. For advanced filtering rspamd provides improved statistics module (based on OSB-Bayes algorithm) and fuzzy hashes database that is generated based on honeypots traffic.
Rspamd can easily be extended with your own rules and plugins that could be written in lua language. Moreover, rspamd ships with numerous lua plugins and rules. Extensive lua API documentation is also available.