Greylisting is a spam filter that temporarily says "not now, try again" to senders it does not recognise. This guide explains how the deferral works, why it delays some mail, and why a greylisting server should make an email verifier return unknown rather than invalid.
Greylisting is an anti-spam technique where a receiving mail server temporarily rejects mail from an unfamiliar sender with a 4xx "try again later" response. Legitimate servers retry and are accepted; many spam sources never retry, so they are filtered out.
The receiving server tracks a "triplet" - the sending IP, the envelope sender, and the recipient - and treats first-time triplets with suspicion:
When a triplet is seen for the first time, the server replies with a temporary 4xx code (commonly 450) instead of accepting the message.
It notes the sender IP, from address, and recipient, and starts a short timer during which a retry will be accepted.
Real mail servers are built to retry deferred mail. On the retry (after the delay window), the message is accepted.
Once a triplet has retried successfully, the server usually remembers it and delivers future messages without delay.
4xx
5xx
The distinction matters: a verifier that reads a 4xx greylist deferral as a 5xx failure will wrongly mark real mailboxes invalid. See how the full pipeline handles this in how email verification works.
An SMTP probe against a greylisting server gets a temporary 4xx code, not a confirmation. The honest response is to classify the address as unknown and re-probe later - never to guess invalid. BounceZero re-probes greylisted domains from its retry queue, so a cautious server does not cost you real addresses.
This is the same principle behind our deliverability guidance: temporary signals are handled as temporary, not treated as permanent verdicts.
Greylisting is a spam-filtering method where a mail server temporarily defers mail from a sender it has not seen before, replying with a 4xx "try again later" code instead of accepting or rejecting it. Real senders retry and get through; a lot of spam software never retries, so it is filtered out.
Usually a few minutes to under an hour. The receiving server holds the first attempt, and the sending server retries on its normal schedule (often 5-15 minutes). After a successful retry the sender is typically whitelisted, so later messages arrive without delay.
No. A bounce is a permanent failure (a 5xx code) meaning the message will not be delivered. Greylisting is a temporary deferral (a 4xx code) meaning "not now, try again". A message that is greylisted is not rejected - it just arrives a little later.
During an SMTP probe, a greylisting server answers with a temporary 4xx code rather than confirming whether the mailbox exists. An honest verifier records this as unknown (or retries) rather than guessing invalid. BounceZero treats a greylist deferral as unknown and re-probes, so a real mailbox is never marked invalid just because the server was cautious.
Greylist deferrals are re-probed, not guessed. 100 free verifications, no card required.
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