451: message closely resembles spam
GMX deferred this message because it looks very much like known spam. Check your system for abuse, and sign with an aligned DKIM domain to improve its standing.
- 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 r0112 is a content judgement. GMX compared the message with spam it has seen and found a high similarity, so it rejected it for now with a 451. If you run the sending infrastructure, GMX first asks you to make sure no spam is leaving it and that it is protected against unauthorised access. It adds that DKIM signing with a domain aligned to the From address has a positive effect on deliverability. If your mail is already signed and you believe it was misjudged, GMX offers the form on the case page, asking for the full error, your DKIM domain and your IP addresses.
Common causes
- The message shares wording, links or structure with spam campaigns GMX has seen.
- Spam is leaving your system alongside legitimate mail and colours its traffic.
- The message is not signed with an aligned DKIM domain, which leaves GMX less to go on.
How to fix it
- 01Check your server for unauthorised use and stop any spam it sends.
- 02Sign the mail with DKIM using a domain aligned to the From address.
- 03Review the content for traits common in spam, such as link shorteners or image-only messages.
- 04If the message is legitimate and already signed, report the misclassification through the case page form.
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 r0112: 451 Reject due to policy restrictionspostmaster.gmx.net
Related codes
- 421Too much of your mail is spam.GMX is refusing mail from your server for a while because too large a share of its recent mail was classed as spam by the GMX filters.
- 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.
- 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