What is greylisting?

Greylisting is an anti-spam technique in which a receiving server answers the first delivery attempt from an unfamiliar sender with a temporary 4xx error. A properly built mail server retries a few minutes later and is then accepted; most spam software never retries. The receiver remembers the sender by the triplet of connecting IP, envelope sender and recipient.

How the exchange looks

C: RCPT TO:<user@example.net>
S: 451 4.7.1 Greylisted, please retry in 300 seconds

The sender’s queue retries. On the second attempt the triplet is known and the message is accepted. From then on that triplet is trusted for a period, typically weeks, so the delay happens once per sender.

The trade-off

Greylisting delays first contact from a new correspondent by a few minutes. That is acceptable for most organisations and unacceptable for some (password-reset mail, time-critical alerts). Two mitigations: exempt senders that pass SPF for a reputable domain, and keep the whitelist of known-good sending ranges current.

Large providers send retries from a different IP than the first attempt. Greylisting implementations therefore match on the /24 network rather than the exact address.

Greylisting in Spamjadoo

Greylisting is one of the handshake stages Spamjadoo can apply per domain and per user, after DNSBL and reverse DNS checks have already removed the bulk of bot traffic. It is off for recipients with a strict-latency class of service and on by default for consumer mailboxes.

Last reviewed 10 September 2026 by Spamjadoo engineering.