451: low DKIM domain reputation
GMX deferred your mail because unusually high traffic arrived from a DKIM signing domain with a low reputation at GMX.
- 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
GMX tracks the reputation of the domains that sign mail with DKIM. Cases r0102 and r0113 mean that a burst of mail arrived signed by a domain GMX rates poorly, and acceptance was deferred with a 451. The signature itself is valid; what holds the mail back is the history of that domain combined with the volume. GMX asks you, or your system manager, to contact it through the form on the case page and to include the list of DKIM signing domains you use, without which it may not be able to handle the request.
Common causes
- Mail signed with your DKIM domain drew spam complaints or spam classifications at GMX.
- The domain is new and sent a large volume before building any history.
- The signing domain is shared, for example a sending service domain, and others damaged it.
How to fix it
- 01Lower the volume to GMX and raise it gradually as the deferrals stop.
- 02Sign with your own domain rather than a shared one.
- 03Send only to recipients who asked for your mail, and remove those who complain or never engage.
- 04Contact GMX through the case page form and list every DKIM signing domain you use.
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 r0102: 451 Reject due to policy restrictionspostmaster.gmx.net
- GMX Postmaster case r0113: 451 Reject due to policy restrictionspostmaster.gmx.net
Related codes
- 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.
- 451Poor reputation of your network area.GMX deferred your mail because unexpected volume arrived from a network area with a negative reputation. An automatic check of your DKIM domain follows.
- 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