What is greylisting?
Greylisting is an anti-spam technique where a mail server temporarily rejects messages from senders it doesn't recognize, returning a 'try again later' response. Legitimate servers retry and get through; many spam systems don't bother, so they're filtered out.
How greylisting works
When an unfamiliar sender attempts delivery, a greylisting server responds with a temporary 4.x.x deferral instead of accepting the message. It records the sender IP, sender address, and recipient as a triplet.
A standards-compliant mail server treats the deferral as a soft bounce and retries after a delay. On the retry, the greylisting server recognizes the triplet and accepts the message. Spam tools that fire once and move on never complete the retry, so their mail is blocked.
How greylisting affects verification
During SMTP verification, a greylisting server may defer the probe with a temporary error, which can look like 'undeterminable' rather than a clear accept or reject. A verifier that treats the first deferral as a failure will misclassify the address.
Good verification handles this by recognizing temporary responses and retrying appropriately, distinguishing a greylisting deferral from a genuine rejection. Mailbeam accounts for greylisting so deferrals don't produce false negatives.
Avoiding greylisting delays for your own mail
To minimize greylisting friction for your campaigns, authenticate with SPF, DKIM, and DMARC, send from consistent IPs, and warm those IPs gradually so receiving servers build familiarity with you.
Greylisting mainly causes a delay on first contact rather than a permanent block, but consistent authentication and sending behavior reduce how often you're deferred.
In practice
Mailbeam probes a mailbox and the server replies 451 greylisted, try again later. Rather than marking the address undeliverable, Mailbeam recognizes the temporary deferral and retries, then reads the real accept/reject response — avoiding a false negative on a perfectly valid address.
Frequently asked questions
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