SpamAssassin

What it means

Apache SpamAssassin describes itself as an open-source anti-spam platform that gives administrators a filter to classify email and block spam. It runs a large set of tests on headers and body: text rules, Bayesian filtering, DNS blocklists and collaborative filtering databases. Each test that matches adds points, and the total is compared with required_score.

The default required_score is 5. The documentation calls that quite aggressive, suitable for a single user, and suggests an ISP might set 8.0 or 10.0, so the same message can be spam on one server and clean on another. It also advises against automatically deleting messages that score as spam.

A SpamAssassin report lists each rule that matched with its points, so running a test message through it shows which rules your content or setup triggers.