SpamAssassin

Apache SpamAssassin is a mature, widely deployed email spam filtering platform that uses a combination of rule-based scoring, Bayesian statistical analysis, DNS blocklists, and collaborative filtering to identify spam messages. It has been the default spam filter on cPanel servers for over a decade and remains one of the most recognized anti-spam tools in the hosting industry.

SpamAssassin works by analyzing incoming emails and assigning a spam score based on hundreds of rules. Each rule tests for specific spam characteristics: suspicious headers, known spam phrases, URL patterns, sender reputation, SPF/DKIM failures, and more. If the cumulative score exceeds a configurable threshold (typically 5.0), the message is marked as spam.

The Bayesian learning system improves accuracy over time by learning from messages that users mark as spam or not-spam. On hosting servers, this can be trained per-user, allowing the filter to adapt to each customer's email patterns. The auto-learn feature trains the Bayesian classifier automatically based on high-confidence spam and ham scores.

SpamAssassin integrates with Postfix and Dovecot through various methods: as a milter (mail filter), through Amavis (a content filter framework), or via spamc/spamd client-server architecture. On cPanel servers, it is pre-configured and customers can enable or disable it and adjust the spam threshold from their cPanel interface.

While SpamAssassin remains effective, newer alternatives like Rspamd offer better performance and more modern features. Rspamd is written in C (versus SpamAssassin's Perl), uses less memory, processes messages faster, and includes features like neural network classification and Redis integration out of the box. Many hosting providers are migrating to Rspamd, though SpamAssassin remains the default on most cPanel installations.