Bounce Codes
451 4.3.0 Temporary Failure Local error in processing / greylisting
A generic temporary failure on the receiving server's side — very commonly the signature of greylisting, where an unrecognized sender is asked to retry later as an anti-spam measure.
Common causes
- Greylisting: receiving server doesn't yet recognize this sender/recipient/IP combination.
- Transient internal error on the receiving mail server.
What to do
Retry after several minutes. Greylisting specifically expects and rewards a retry — treating this as a final failure produces false-negative results for perfectly valid addresses.
FAQ
How is this different from a 450?
In practice the two codes are often used somewhat interchangeably by different mail server software for similar temporary conditions; the enhanced code and surrounding text (if any) usually clarify whether it's a mailbox-specific or server-wide issue.
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