Mimecast rejection error (Mimecast): The message was rejected
The recipient's gateway is Mimecast and it refused the message - for IP reputation, failed authentication, a policy set by that company, or content.
How to fix Mimecast rejection
- Read the exact text after 'Mimecast' in the bounce - it names the policy.
- Check SPF/DKIM/DMARC, blacklists and content; if all clean, ask the recipient's IT to allow-list you.
Why this happens
Receivers score every sending IP and domain. A history of complaints, spam-trap hits, high bounce rates or a listing on a blocklist pushes that score down until mail is deferred or refused outright.
How to keep it from coming back
Warm up new domains and IPs gradually, send only to verified and relevant recipients, keep complaints near zero, and check your IPs and domains on blocklists regularly. Spread volume across mailboxes and domains instead of pushing one harder.
Should I retry a Mimecast rejection error?
Not as it stands. A permanent error means the same message will be refused again until the cause is fixed on your side. Fix the cause first, then send a fresh message; if it keeps failing after that, check the full text of the bounce for the exact reason.
Got the full bounce message?
The text after the code names the exact reason, and the same code can mean different things at different receivers. Paste the whole message into the decoder - it runs in your browser and nothing is sent.
Open the bounce decoderRelated error codes
Seeing this again and again? That's usually a reputation or authentication problem we can trace and fix at the source.
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