451: transaction limit per session reached
Your server sent more messages over one SMTP session than GMX allows, currently 100, so GMX deferred the next one. Close the session and open a new one.
- 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 Transaction limit reached
What it means
GMX caps how many transactions (messages, each starting with MAIL FROM) one SMTP session may carry, as protection against attacks and against faulty remote systems. When your server reaches the cap, GMX answers with a 451 and, citing RFC 5321 section 3.8, may close the connection. The case page gives the current limit as 100 transactions per session, warns that it may change, and rules out exceptions. Your server should close the connection, open a new one and repeat the transaction that was refused.
Common causes
- Your server keeps one session open and sends more than 100 messages through it.
- Connection reuse is set to an unlimited number of messages per session.
How to fix it
- 01Limit your server to fewer than 100 messages per session toward GMX.
- 02Make sure a 451 in mid-session leads to a new connection and a retry of that message.
- 03Plan for the limit to change, rather than setting your server exactly at 100.
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 r0406: 451 Transaction limit reachedpostmaster.gmx.net
Related codes
- 452Too many recipients in one message.Your server named more recipients in one transaction than GMX accepts, currently 20 per message, so the rest were deferred. Split the recipients into several transactions.
- 421Too many connections from your IP.Your server opened more simultaneous connections to a GMX mail server than one IP may hold. Wait for an open connection to close, then connect again.
- 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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