451: missing 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.
- 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 requires a valid DKIM signature, and the signing domain has to match the domain in the From header (RFC 5322.From), at least in relaxed alignment, where a parent domain and a subdomain count as a match. These cases combine that with volume: unusually high traffic arrived with no DKIM signature (r0101), or with a signature from an unaligned domain (r0103, r0110), or a domain that normally signs stopped doing so (r0105). GMX defers with a 451. Its case pages recommend strict alignment where possible, so that DKIM, From and envelope domains all match.
Common causes
- Your mail is not DKIM signed at all.
- Your mail is signed with the domain of your sending service rather than your own.
- DKIM signing stopped after a change of server, provider or DNS.
- The volume to GMX rose sharply at the same time.
How to fix it
- 01Sign every message with DKIM using a key published under your own domain.
- 02Make the DKIM domain match the From domain, either exactly or as parent and subdomain.
- 03If you send through a provider, set up custom DKIM for your domain there.
- 04Send a test message and confirm that the DKIM result is pass and the domain aligns.
- 05If deferrals continue, contact GMX through the case page form with the full error and the DKIM domain you sign with.
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 r0101: 451 Reject due to policy restrictionspostmaster.gmx.net
- GMX Postmaster case r0103: 451 Reject due to policy restrictionspostmaster.gmx.net
- GMX Postmaster case r0105: 451 Reject due to policy restrictionspostmaster.gmx.net
- GMX Postmaster case r0110: 451 Reject due to policy restrictionspostmaster.gmx.net
Related codes
- 451Low DKIM domain reputation.GMX deferred your mail because unusually high traffic arrived from a DKIM signing domain with a low reputation at GMX.
- 554Rejected under the sender domain's DMARC policy.The message failed SPF and DKIM as DMARC requires, and the From domain publishes p=reject, so GMX rejected it as the domain owner asked.
- 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