What it looks like in your logs
451 4.3.0 Temporary server error. Please try again later. [203.0.113.10] - mx.example.comWhat does 451 mean?
RFC 5321 describes 451 as a requested action aborted because of a local error in processing. Local here means local to the receiving server: something on its side failed while it was handling your transaction. It is a temporary reply, so your server should queue the message and retry rather than giving up. In other words, you did nothing wrong in that transaction, and resending by hand is rarely needed. Let the queue do its job, and only investigate if the same domain keeps answering 451 for hours.
Typical culprits are a DNS lookup that timed out while the receiver was checking your SPF or DKIM, an anti-spam engine that crashed or stalled, a database or directory lookup that failed, or a server that simply ran short of resources. Some greylisting setups also answer with 451 instead of 450. Most 451s resolve within minutes to a few hours.
Gmail returns 451 4.3.0 for transient processing problems and for some protocol issues, such as sending to several destination domains in one transaction. Microsoft 365 uses 451 with 4.7.x codes, for example 451 4.7.500 to 4.7.699 when it temporarily limits a sending IP, and "451 4.7.1 Please try again later" for greylisting-style deferrals.
Soft or hard bounce?
| Soft bounce (4xx) | Hard bounce (5xx) | |
|---|---|---|
| Nature | Temporary | Permanent |
| Sender behaviour | Queue and retry with backoff | Stop, suppress the address |
| Reputation risk | Low, unless it repeats for days | High above a 2% bounce rate |
| 451 | ✓ this code |
Common causes
- The receiver could not complete a DNS lookup for your SPF, DKIM or DMARC record in time.
- An anti-spam or anti-virus engine on the receiving server failed or timed out.
- A directory or database lookup for the recipient failed temporarily.
- The receiving server is short on CPU, memory or worker processes.
- A greylisting or reputation policy deferred your message using 451 instead of 450.
How to fix 451
- Let the MTA retry on its normal schedule; most 451 errors clear on their own.
- If 451s repeat for one provider, confirm your SPF and DKIM records resolve quickly with the SPF checker and DKIM checker.
- Check that your DNS host is not having an outage or rate limiting large resolvers.
- Read the enhanced code: 4.3.x points to the receiving system, 4.7.x points to a policy or reputation limit.
- If a single domain returns 451 for more than 24 hours, contact its postmaster through another channel.
451 questions
What does SMTP error 451 mean?
It means the receiving server ran into a temporary problem of its own while processing your message and aborted the transaction. The message stays in your queue and is retried. It is not a sign that the recipient address is bad, and it usually clears within minutes or hours.
How long should I wait before retrying after a 451?
Your MTA handles this automatically. Typical schedules retry after 5 to 15 minutes, then back off to longer intervals, and keep trying for up to several days. You do not need to resend by hand. If the same domain still returns 451 after a day, investigate DNS or contact the recipient.
Can my own DNS cause a 451 at the receiver?
Yes. When the receiver cannot fetch your SPF, DKIM or DMARC records because your DNS is slow or failing, it may defer with 451 rather than decide. Check your records with the SPF checker and review our guide to DNS records for email to make sure they are published correctly.