What is hash-based filtering?

Hash-based filtering creates mathematical signatures from email content that allow providers to quickly identify known spam patterns. When a message arrives, the system generates a hash and compares it against a database of flagged signatures.

This approach efficiently catches spam campaigns that reuse templates. Even if the spammer changes sender addresses or domains, the content hash reveals that the message matches a known bad pattern.

Legitimate senders can trigger hash filters if their templates closely resemble content that was previously mass-reported. Using common industry templates or purchasing pre-made designs increases this risk.

Hashing is like a fingerprint database for cargo manifests. It does not matter which ship delivers the goods. If the manifest matches known contraband documentation, the shipment gets flagged.