ModSecurity - ModSecurity is an Open Source web application firewall developed by Trustwave's SpiderLabs.
SpamAssassin - Use this program to "Unspam" your mailbox(es).
Shadow Daemon - Shadow Daemon is a collection of tools to detect, record and prevent attacks on web applications.
Rspamd - Rspamd is an advanced spam filtering system that allows evaluation of messages by a number of rules...
ironbee - Universal web application firewall engine
Oikik Antispam - Oikik is a cloud-based email filtering service that works with your existing email service.