Glossary
SMTP 451
A transient failure meaning the requested action was aborted because of a local error while the receiving server processed the command.
In plain terms
Something went wrong on their end, not yours. Try again later.
Why it matters for deliverability
451 is frequently used for greylisting and for temporary policy holds, so it is worth reading the accompanying text rather than treating every 451 the same way.
What to do about it
- Retry on a normal backoff schedule.
- If 451s cluster at one provider, read the response text — greylisting and reputation holds both live here.
- Never suppress an address on a 451 alone.
Source
RFC 5321 — Simple Mail Transfer Protocol
“451 Requested action aborted: local error in processing”
Checked 2026-08-29
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