451: suspected replay attack
GMX deferred your mail because inconsistent details in the delivery, combined with high traffic, suggested a replay attack. Report it if you think this is a mistake.
- 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
A replay attack takes a genuine, signed message and sends copies of it in bulk from other servers, borrowing the original sender reputation. Case r0114 means GMX saw information in your delivery that did not fit together, together with unexpectedly high traffic, and judged a replay likely, so it rejected the mail temporarily with a 451. The case page does not list which details were inconsistent. If you think it is a false positive, GMX asks you or your system manager to use the form on the case page, with the full error, the DKIM domain and the IP addresses you send from.
Common causes
- Signed mail from your domain is being resent in bulk by someone else.
- Your own mail travels in a way that looks inconsistent, such as signed in one place and sent from an unexpected network.
- A large volume arrived at once, which is when GMX applies this check.
How to fix it
- 01Check whether copies of your messages are being sent from servers you do not run.
- 02Sign mail close to where it is sent, and keep DKIM keys out of reach of third parties.
- 03Keep your sending volume steady rather than in sudden bursts.
- 04If it is a false positive, contact GMX through the case page form with the full error, DKIM domain and IPs.
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 r0114: 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.
- 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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