Greylisting

What it means

Greylisting relies on a difference in behaviour: a real MTA queues and retries after a temporary failure, while much spam software does not. RFC 6647 describes the variants. The receiver records the connecting IP, often with the envelope sender and recipient, answers the first attempt with a 4xx reply, and accepts a retry that arrives after a minimum delay. Senders that pass are usually remembered for a while.

For a legitimate sender the cost is a delay on first contact. RFC 6647 notes a known problem: when retries leave from a different outbound server or IP each time, the receiver sees a new sender on every attempt and greylists again. A first message that arrives late, after a 451 or similar reply in the logs, and is then delivered on retry was most likely greylisted.