SMTP 451 4.7.1
Temporarily deferred (greylisting or policy)
451 4.7.1 means the receiving server has deferred your message for now, most often because of greylisting or a temporary policy check. It is a soft bounce. A properly configured sending server retries after a few minutes and the message usually goes through. It only becomes a problem if retries keep failing.
Should you remove the address?
Keep the address; fix the sending side
Keep it. The address hasn't been judged at all; the server only asked you to try again.
Soft bounce. Your sending server retries on its own. Investigate only if it keeps failing.
What causes it
- The receiving server greylists mail from senders it hasn't seen before.
- A temporary policy or reputation check deferred the message.
- The sending IP changed between attempts, so greylisting started again.
How to fix it
In order — try the first one first.
- Make sure your sending tool retries deferred mail. Tools that treat any 4xx as final will lose greylisted messages.
- If it keeps failing for hours, read the text; some servers use 451 4.7.1 for blocklist or reputation checks.
- Send from a consistent IP. Greylisting tracks the sending IP, so rotating IPs restarts the delay.
Who sends it
Many corporate and self-hosted servers and filtering gateways. Gmail, Microsoft and Yahoo don't list 451 4.7.1 in their published tables.
Gmail, Microsoft and Yahoo don't publish wording for this code, so none is quoted here. The text in your own bounce is the best guide to the exact cause.
Common questions
- What does SMTP 451 4.7.1 mean?
- 451 4.7.1 means the receiving server has deferred your message for now, most often because of greylisting or a temporary policy check. It is a soft bounce. A properly configured sending server retries after a few minutes and the message usually goes through. It only becomes a problem if retries keep failing.
- Should I retry a 451 4.7.1 error?
- Yes, and your sending server does it automatically. 451 is a temporary deferral, and legitimate mail usually arrives on a retry within minutes or hours. Only investigate if it persists across many retries.
- How do I fix SMTP 451 4.7.1?
- Make sure your sending tool retries deferred mail. Tools that treat any 4xx as final will lose greylisted messages.
- Is 451 4.7.1 a hard bounce or a soft bounce?
- 451 4.7.1 is a soft bounce. It is temporary, and your sending server retries it automatically. Your sending server retries on its own. Investigate only if it keeps failing.
- Should I remove the address after a 451 4.7.1 bounce?
- Keep it. The address hasn't been judged at all; the server only asked you to try again.
- Will verifying my list prevent 451 4.7.1?
- Not for delivery. During a mailbox check, a greylisting server gives the same answer, which is why a verifier marks such an address unconfirmed rather than valid or invalid.
Clean the list before the next send.
A bounce report only tells you about the addresses you already mailed. The rest of the list has the same kinds of dead mailboxes in it, and each one becomes a hard bounce recorded against your domain the moment you send.
- A real SMTP mailbox check on every row, before anything is sent
- Dead and mistyped addresses split out, so they never reach your sending tool
- Catch-all, disposable and role addresses flagged alongside
One plan, cancel anytime, no contract. Sign in with Google and your first 100 email checks are free, no card. The free tools stay free either way.
Other 451 replies
Sources
Provider wording is quoted only where the provider publishes it.
Last reviewed 2026-10-22