Spam filter · 4.x.x transient failure
Every SMTP 451 response on record from Exchange Online Protection, with the verbatim text, the enhanced status code, a severity rating and a plain-English fix.
The receiver aborted the action because of a local error on its side, such as greylisting, a reputation or content check still running, or a short policy hold. Retries normally succeed once the hold clears.
smtp;451 4.7.0 Temporary server error. Please try again later. PRX4 NextHop: example.com [example.com]
You are sending faster than this receiver currently accepts from you. Slow down and the automatic retry will deliver the queued mail.
smtp;451 4.7.500 Server busy. Please try again later from [x.xx.xx.x]. (AS77712344) [#.prod.protection.outlook.com]
You are sending faster than this receiver currently accepts from you. Slow down and the automatic retry will deliver the queued mail.
The receiver aborted the action because of a local error on its side, such as greylisting, a reputation or content check still running, or a short policy hold. Retries normally succeed once the hold clears. At Exchange Online Protection, it is a temporary deferral that is retried automatically, and the specific reason is in the enhanced status code and message text of each response below.
Match your bounce text to one of the responses below and follow its guidance. Recipient-side wording means fix the address. Authentication or reputation wording (SPF, DKIM, DMARC, blocklist) means fix your sending setup, and check Exchange Online Protection's postmaster documentation.
