Greylisting

What Greylisting means

A greylisting server records a triplet — connecting address, envelope sender, recipient — the first time it sees one, answers with a 4xx reply and refuses the message. A conforming sender queues it and tries again; when the retry arrives the triplet is known and the message is accepted. The practice is described in RFC 6647.

The cost is latency on the first message between any two parties: anything from a few minutes to a full retry interval. RFC 5321 recommends that a sender wait at least 30 minutes before its first retry, so a password reset or a sign-up confirmation can arrive long after the reader has given up and asked for another one.

For a sender, greylisting is not a fault to fix but a delay not to mistake for one. Confirm that your outbound server really does retry, that it retries from an address your SPF record covers — a pool that rotates addresses between attempts can keep failing the triplet test — and that the text of the 4xx reply describes greylisting rather than a rate limit or a blocklist rejection wearing a temporary code.