What is a greylisting?
Greylisting is an anti-spam technique where a mail server temporarily rejects a message from an unfamiliar sender with a 4xx code, on the assumption that real mail servers retry and spam software does not. Legitimate mail arrives minutes later on the retry; the first attempt always fails.
For verification, greylisting is a persistent nuisance. A mailbox probe is a single connection with no retry — it cannot wait twenty minutes — so a greylisting server returns an inconclusive result and the address is correctly reported as risky rather than invalid.
The rate of greylisting against a verification host depends on how well its identity is configured. A host whose HELO name, reverse DNS and SPF record all agree gets greylisted far less often than one where they do not, which is why the reverse DNS on a verification server is a deliverability setting rather than a cosmetic one.
Last reviewed 2026-09-11