How do blocklists detect spam?

Blocklists use multiple detection methods. Spam traps are addresses that should never receive legitimate email; messages to traps indicate purchased lists or scraping. High complaint rates signal recipients marking messages as spam.

Automated systems analyze: sending patterns (sudden volume spikes), content characteristics (spammy templates), network behavior (probing for open relays), and known spam infrastructure fingerprints. Human review validates automated detections for major listings.

Honeypots attract spammers with fake vulnerabilities, capturing their techniques and sources. Shared intelligence from MBPs and security researchers feeds additional detection. The combination makes blocklists difficult to evade while maintaining accuracy for legitimate sender classification.