Temporary — will retry
SMTP 451
local error in processing
SMTP 451 means the receiving server aborted the request because of an error on its side. RFC 5321 calls it "requested action aborted: local error in processing". It is temporary, so your server retries. Many servers also use 451 for greylisting and for temporary spam or policy holds.
How it appears
451 Requested action aborted: local error in processing
451 4.3.0 Mail server temporarily rejected message.
451 4.7.1 Please try again later
What causes it
- The receiving server hit an internal error, such as a failed database lookup or a misconfigured filter.
- Greylisting: the first attempt from a new sender is deferred on purpose.
- A temporary reputation hold on your IP or domain.
- The recipient's server could not reach its own directory or spam-scanning service.
How to fix it
In order — try the first one first.
- Let your server retry on its normal schedule.
- If the deferral persists past a day, read the enhanced code: 4.3.0 is a generic server problem, 4.7.x is policy or reputation.
- If only one recipient domain returns 451 for everyone, the problem is probably theirs. Contact their postmaster if the mail is important.
Common questions
- What does SMTP 451 mean?
- SMTP 451 means the receiving server aborted the request because of an error on its side. RFC 5321 calls it "requested action aborted: local error in processing". It is temporary, so your server retries. Many servers also use 451 for greylisting and for temporary spam or policy holds.
- Should I retry a 451 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?
- Let your server retry on its normal schedule.
Specific 451 replies
Last reviewed 2026-09-11