What’s “greylisting” and why is it used?
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Greylisting is a spam filtering technique that temporarily rejects the first delivery attempt from an unknown sender. The receiving server returns a 4xx temporary error and expects the sender to retry later.
Legitimate mail servers follow SMTP standards and automatically retry failed deliveries, typically after a few minutes. Spam systems, especially botnets and bulk mailers, often do not implement proper retry logic. They move on to the next target rather than waiting.
When the legitimate sender retries, the greylisting system recognizes the combination of sender IP, sender address, and recipient address. It allows the message through and remembers that tuple for future deliveries.
Greylisting is effective but causes delivery delays for first-time senders. Many organizations use it as one layer among many, and some have abandoned it as spammers have adapted.
Greylisting is the harbor saying: come back in an hour and prove you are serious. Real merchants return. Pirates do not.
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