451: DKIM domain suspended for spam
GMX found a massive amount of spam signed with your DKIM domain and sent through your mail system, and has suspended acceptance of mail from them.
- 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
- The sender. The fix is on the sending side: your domain, your server or your list.
- What the server says
451 Reject due to policy restrictions
What it means
Case r0409 is serious even though its reply starts with 451. GMX determined that a large volume of spam reached its customers through the mail system you use, DKIM signed with your domain. To stop more of it, GMX suspended the privileges your system and domain had and refused further mail. Retries will not get through on their own. GMX asks you to take every step needed to stop the misuse, then contact it through the form on the case page with the full error, the DKIM domain you sign with and your IP addresses, ranges and AS.
Common causes
- Accounts on your system were compromised and used to send spam with your DKIM signature.
- A customer or user of a shared system sent spam signed with your domain.
- Your DKIM private key leaked and is being used elsewhere.
How to fix it
- 01Find the source of the spam and stop it: compromised accounts, abusive users, exposed scripts.
- 02If you suspect the DKIM key leaked, publish a new key and retire the old one.
- 03Hold back mail to GMX addresses until the cause is removed.
- 04Contact GMX through the case page form with the full error, your DKIM domain and your IPs, ranges and AS.
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 r0409: 451 Reject due to policy restrictionspostmaster.gmx.net
Related codes
- 554System and domain suspended for spam.GMX permanently refused your mail after a massive amount of spam reached its customers through your mail system and domain, and suspended their privileges.
- 451Low DKIM domain reputation.GMX deferred your mail because unusually high traffic arrived from a DKIM signing domain with a low reputation at GMX.
- 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.
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