451: mimecast does not recognise either domain
Mimecast saw a message where neither the sender’s nor the recipient’s domain belongs to its customer, so it refused to pass it on, as relaying for outsiders is not allowed.
- Mimecast
- Provider
- Temporary (4xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 451
- Provider
- Mimecast
- Temporary or permanent
- TemporaryThe sending server keeps the message and tries again later.
- Who has to act
- The recipient's side. Their mail admin has to change something; you can tell them.
- What the server says
451 An open relay is not allowed.
What it means
Mimecast only carries mail for domains its customers have registered with it, plus outbound mail from servers a customer has authorised. When both the sender and the recipient domain in a transaction are external to Mimecast, or the connecting IP is not recognised as an authorised one, the message looks like an attempt to use Mimecast as an open relay and is refused. For an outside sender this usually means the recipient domain’s MX records point to Mimecast while Mimecast does not hold that domain for any customer account. Although the code is temporary, retries will keep failing until the configuration changes.
Common causes
- The recipient domain’s MX records point at Mimecast, but the domain is not, or is no longer, set up in the company’s Mimecast account.
- A company’s own server is sending outbound mail through Mimecast from an IP that is not registered as an authorised outbound address.
How to fix it
- 01Check the spelling of the recipient’s domain.
- 02If the address is right, tell the recipient’s IT team that their domain’s mail routing through Mimecast is refusing external senders.
- 03If you are the Mimecast customer, contact Mimecast support to add the domain or the authorised outbound address, as Mimecast advises.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
- Policies - Mimecast SMTP Error Codes (Mimecast support)mimecastsupport.zendesk.com
Related codes
451 at other providers
- 451Sender domain could not be resolved.Yahoo could not confirm that the domain in your envelope sender or From address exists. As a 451 it is temporary; Yahoo can also return the same problem as a permanent 554.
- 451Temporary reject for misuse or guideline violations.GMX deferred your mail with a 451 because of an exceptional blocklist or a breach of its sending guidelines. The message is retried, but the cause has to be removed first.
- 451Transaction limit per session reached.Your server sent more messages over one SMTP session than GMX allows, currently 100, so GMX deferred the next one. Close the session and open a new one.
- 451Far more mail than expected.GMX received far more mail than it expected from your system, your DKIM domain or your network, and deferred the rest for now. A higher limit can be requested.
- 451Missing or unaligned DKIM under high traffic.GMX deferred your mail because it arrived in unusual volume without a DKIM signature that matches the From domain. Sign with an aligned DKIM domain to clear it.
- 451Low DKIM domain reputation.GMX deferred your mail because unusually high traffic arrived from a DKIM signing domain with a low reputation at GMX.
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