What is greylisting?
Also called: graylisting
Greylisting is an anti-spam technique where a mail server temporarily rejects mail from an unfamiliar sender with a 4xx code, on the assumption that real mail servers retry and much spam software does not. Legitimate mail arrives on the retry minutes later; the first attempt always fails.
Greylisting is described in RFC 6647. The server usually remembers the combination of connecting IP, sender and recipient, and accepts the same combination once enough time has passed.
For verification it is a nuisance. A mailbox probe cannot sit and wait for the greylist window to expire, so a greylisting server gives an inconclusive answer and the address should be reported as unconfirmed rather than invalid.
How often a verification host is greylisted depends partly on how well its identity is set up. A host whose HELO name, reverse DNS and SPF record agree looks like a real mail server, which is what greylisting is trying to detect.
Example
451 4.7.1 Greylisted, please try again laterHow SimpleVerifier handles it
A greylisting reply is a temporary refusal, so the address comes back risky · Server would not confirm rather than invalid.
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Sources
By SimpleVerifier Team · Last reviewed 2026-10-13 · All 50 glossary terms