Greylisting is a spam-control method where a receiving server answers a first delivery attempt from an unfamiliar sender with a temporary 4xx error. Proper mail servers queue and retry; many spam tools do not.
Why it matters
The server remembers the combination of sending IP, envelope sender and recipient, and accepts the retry after a short delay, often a few minutes. It is cheap and effective but delays first-time mail, which is noticeable for signup confirmations, password resets and one-time codes. Large providers that send from many rotating IPs can also cause repeated deferrals because each retry comes from a different address.
Example
A typical log and reply:
RCPT TO:<jane@example.com>
451 4.7.1 Greylisted, please try again in 300 secondsThe sender's MTA keeps the message in its queue and retries, perhaps at 5, 15 and 30 minutes. After the retry succeeds, the sender triple is allowed for a period and later mail passes immediately.
Common misconceptions
- A greylist response is a failure. It is a deliberate temporary deferral and is not a bounce unless retries stop.
- Greylisting blocks all spam. Spammers using real queues, or compromised servers, retry normally.
- It is harmless for time-sensitive mail; a short delay can break one-time login codes.
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