451 SMTP bounce: local processing error
What 451 means
The receiving server hit an internal processing error during the transaction. It is temporary, so the message should be retried.
Message text
Requested action aborted: local error in processing
Common causes of 451
- Internal error, filter or database failure on the receiving server
- DNS lookups needed for a receiving-side check failed (for example SERVFAIL or timeout)
How to fix 451
- Wait and let the queue retry.
- If it persists, check that your domain's DNS answers reliably with MX lookup and SPF checker, since DNS failures during checks can produce 451.
- Confirm your MX records point at live hosts with MX lookup.
Check your domain
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How to read 451
The first part, 451, is the SMTP reply code. Requested action aborted, local error in processing: the server hit a problem and did not finish the command. Replies that start with 4 are temporary and replies that start with 5 are permanent (RFC 5321 §4.2.1).
This reply carries no enhanced status code. The three-digit reply code is the only machine-readable signal, so read the text that follows it to learn the reason.
Sending servers should retry a deferred message. RFC 5321 §4.5.4.1 advises waiting at least 30 minutes between attempts and trying for at least 4 to 5 days before giving up and returning a bounce. If the deferral keeps repeating, fix the cause rather than waiting.
Official source: www.rfc-editor.org. Last verified 2026-10-20.
Related guides
- How to create an SPF record (syntax and examples)
- How DKIM works and how to publish a DKIM record
- How to set up a DMARC record, from p=none to p=reject
Related codes
- 421 SMTP: service not available
- 450 SMTP: mailbox unavailable
- 452 SMTP: insufficient storage
- 4.2.2 SMTP: mailbox full
- 4.4.1 SMTP: no answer from host
- 4.7.0 SMTP: security status
FAQ
What does 451 mean?
The receiving server hit an internal processing error during the transaction. It is temporary, so the message should be retried. This is a temporary failure.
How do I fix 451?
Wait and let the queue retry. If it persists, check that your domain's DNS answers reliably with MX lookup and SPF checker, since DNS failures during checks can produce 451. Confirm your MX records point at live hosts with MX lookup.
Is 451 permanent, and should I retry?
451 is a temporary failure. Your mail server normally retries by itself; if it keeps repeating, fix the cause.