What are heuristic filters?

Heuristic filters apply predefined rules that encode expert knowledge about spam characteristics. Each rule tests for a specific pattern, and matching rules add points to a spam score.

Examples of heuristic rules include: subject line contains "FREE" in all caps, message has more images than text, HTML includes hidden text, links use URL shorteners, or headers show signs of forgery. Each rule carries a weight based on its reliability.

SpamAssassin is the most famous heuristic filtering system, with hundreds of rules developed over two decades. These rules are regularly updated as spam tactics evolve.

Heuristics are transparent and explainable. When a message is flagged, you can see exactly which rules it triggered. This makes troubleshooting easier compared to black-box ML models.

Heuristic filters are the experienced inspector's checklist, codified wisdom from years of watching what spammers do.