SMTP error 451
Requested action aborted — local error in processing.
Updated Jul 1, 2026
A temporary local error on the receiving side aborted the message — often greylisting, a content or anti-spam check, or an overload. Retry later; if it keeps happening it is usually a reputation or filtering issue worth investigating.
Retry; if it persists it is usually a reputation or filter issue worth investigating.
What it means
451 means the receiving server hit a problem while processing your message and gave up for now. Because it is a 4xx code the message is not rejected outright — a retry frequently succeeds, which is exactly how greylisting is designed to work.
Common causes
The server deliberately defers a first-time sender and expects a retry. Legitimate senders retry and get through; many spam sources do not.
A filter deferred the message pending a check, or on a soft spam signal.
A transient resource problem on the receiving side.
How to fix it
- Let your queue retry
For greylisting, the second attempt after a short wait usually delivers. Make sure your sender actually retries deferrals.
- Watch for a pattern
If 451 persists past normal retries, it is filtering, not greylisting. Check the response text for spam or policy wording.
- Shore up authentication and reputation
Passing SPF, DKIM, and DMARC and having a warm sending history reduces soft deferrals.
Paired code
A real bounce often shows both a 3-digit code and an enhanced code together. This one commonly pairs with:
Common questions
Is 451 the same as being blocked?
No. 451 is temporary — the server may accept the message on retry. A permanent block is a 5xx code.