451: temporary DNS error during the MX lookup
GMX could not complete the MX lookup for the sender or recipient domain because of a temporary DNS error, so it deferred the message.
- GMX, WEB.DE
- Provider
- Temporary (4xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 451
- Provider
- GMX, WEB.DE and mail.com
- Temporary or permanent
- TemporaryThe sending server keeps the message and tries again later.
- Who has to act
- Either side, depending on the cause. Start with the checks below that are yours.
- What the server says
451 Requested action aborted: local error in processing
What it means
Before accepting mail, GMX looks up MX records: for the sender domain, so a reply is possible (case r1405), and for the recipient domain in RCPT TO (cases r1406 and r1702). In these cases the lookup failed with a temporary error, such as a timeout or a SERVFAIL, where the domain exists but its name server is not answering. GMX defers with a 451 and your server retries. GMX expects the problem to clear and asks for patience. If it persists for the sender domain, check the DNS infrastructure that serves it.
Common causes
- The domain's name servers are slow, overloaded or unreachable.
- A DNSSEC or delegation fault makes resolvers fail with SERVFAIL.
- A short network fault between GMX and those name servers.
How to fix it
- 01Let your server retry; most of these clear without action.
- 02If it is your sender domain, test its name servers from outside your network and fix slow or failing ones.
- 03If it is the recipient domain and the error lasts, contact the recipient by other means.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
- GMX Postmaster case r1405: 451 Requested action aborted: local error in processingpostmaster.gmx.net
- GMX Postmaster case r1406: 451 Requested action aborted: local error in processingpostmaster.gmx.net
- GMX Postmaster case r1702: 451 Requested action aborted: local error in processingpostmaster.gmx.net
Related codes
- 421Reverse DNS lookup failed for now.GMX could not look up your IP address reverse DNS because of a temporary DNS error, so it deferred the connection. Your server's retry usually succeeds.
- 550Recipient domain does not exist.The domain in the recipient address did not exist in DNS when GMX tried to deliver (NXDOMAIN), so the message could not be delivered.
- 451Local error in processing.The receiving server hit a problem on its own side while handling the message and stopped. The reply is temporary, so the sending server will try again.
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.
- 451Local error in processing.IONOS could not accept the message because the service was temporarily unavailable. IONOS points to slow DNS answers from the sending side as the thing to check.
- 451Greylisting or a reputation check delayed the message.Mimecast is holding off a first contact from your server through greylisting, or because your IP’s reputation is poor. A well-behaved server retries and usually gets through.
- 451Mimecast server at full load.The Mimecast server your message reached was at maximum load and asked your server to try again later. Nothing is wrong with your mail.
- 451Mimecast 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.
- 451Sending IP temporarily blocked for exceeding a limit.Your sending IP went over a limit Mimecast applies, so it is refused for now. Mail is accepted again once your traffic falls back under the limit.
Further reading
All 57 GMX, WEB.DE codesEvery code in the wikiPaste your own bounce