Status 1#
smtp;451 Open relay not allowed - https://community.mimecast.com/docs/DOC-1369#451 [#]
The server is refusing to relay this message for now. Check you are using the right outbound route for the domain.
Spam filter · 4.x.x transient failure
Mimecast SMTP 451 Errors: Every Bounce Response Explained
Every SMTP 451 response on record from Mimecast, 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.
- Spam filter
- Mimecast
- Code
- 451 (4xx)
- Responses
- 3
- Highest severity
- Unclassified
Mimecast SMTP 451 responses
Status 2#
smtp;451 Internal resource temporarily unavailable - https://community.mimecast.com/docs/DOC-1369#451 [#]
The receiver deferred this message and will retry it automatically. A failure that sticks around usually traces back to rate limits, reputation, or a policy on the receiving side.
Status 3#
smtp;451 Invalid Recipient - https://community.mimecast.com/docs/DOC-1369#451 [#]
The receiver could not confirm the recipient address on this attempt. If the retry keeps failing, treat the address as invalid and remove it.
Postmaster & documentation
Mimecast SMTP 451 questions
›What does an SMTP 451 error from Mimecast mean?
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 Mimecast, 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.
›How do I stop Mimecast from returning SMTP 451?
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 Mimecast's postmaster documentation.