451: envelope sender breaks GMX guidelines
GMX deferred the message because the sender information in the envelope (MAIL FROM) breaks its guidelines. Check that the MAIL FROM address is complete and correct.
- 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
The envelope sender is the address your server gives in the MAIL FROM command (RFC 5321). It is where bounces go, and GMX expects it to be complete and correct. Case r0115 means the address your server used broke GMX guidelines, so the message was rejected for now with a 451. The case page does not list the exact rules, only the instruction to check that the MAIL FROM is complete and correct. An incomplete address or a local host name in place of a real domain would both fit that wording.
Common causes
- The MAIL FROM address is missing a domain or uses a local host name.
- An application builds the envelope sender from a placeholder or a typo.
- The envelope domain cannot receive replies or bounces.
How to fix it
- 01Look at the MAIL FROM your server sends (in its logs or the Return-Path header) and correct it.
- 02Use a full address in a domain you own that can receive bounces.
- 03Fix the application or server setting that builds the address, then let the message retry.
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 r0115: 451 Reject due to policy restrictionspostmaster.gmx.net
Related codes
- 550Sender domain has no valid MX or A record.GMX refused the mail because your sender domain is not set up in DNS as the RFCs require: it needs an MX record whose host has a valid A record.
- 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