Greylisting
An anti-abuse practice of returning a temporary failure to senders the receiving server does not recognise, on the assumption that legitimate mail servers retry and many spam sources do not.
In plain terms
A deliberate 'try again in a minute' aimed at senders the server has not seen before.
Why it matters for deliverability
Greylisting delays first contact rather than blocking it, which makes it a common and harmless-looking cause of slow delivery to new recipients. It becomes a real problem only if your sending system does not retry properly.
What to do about it
- Make sure your MTA retries temporary failures rather than dropping them.
- Expect a delay on first contact with a new domain; do not read it as a block.
- Keep sending IPs stable — changing them repeatedly restarts the greylisting clock everywhere.
Source
RFC 6647 — Email Greylisting: An Applicability Statement for SMTP
“the practice of providing temporarily degraded service to unknown email clients as an anti-abuse mechanism”
Checked 2026-08-29
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