421: too 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.
- GMX, WEB.DE
- Provider
- Temporary (4xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 421
- 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
421 Too many connections
What it means
GMX limits how many connections one IP address may hold at the same time. Case r0405 means your server tried to open one more than that. The answer is temporary: your server should wait until one of its existing connections ends, and then connect again, which most mail servers do on their own. GMX notes that the limit is set per mail exchange server, may differ from server to server, and may change at any time, and it makes no exceptions or individual changes. The usual fix is to lower the concurrency your server uses toward GMX.
Common causes
- Your server opens many parallel connections to the same GMX host.
- Several processes or servers behind one IP deliver to GMX at the same time.
How to fix it
- 01Lower the number of concurrent connections your server opens to GMX hosts.
- 02Send several messages over each connection instead of opening a new one per message.
- 03Let the deferred messages retry; GMX will not raise the limit on request.
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 r0405: 421 Too many connectionspostmaster.gmx.net
Related codes
- 451Transaction 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.
- 421Too many parallel connections.Your server opened too many simultaneous connections to the t-online.de mail servers, so the extra ones were refused for now and the mail is delayed.
- 421Service not available.The receiving server is closing the connection and will not take mail right now. The reply is temporary, so the sending server keeps the message and tries again later.
421 at other providers
- 421 [RP-001]Outlook.com rate limit for the sending IP.Outlook.com is limiting how fast your IP can send to it because of the IP or domain's reputation; it is temporary and your server should retry.
- 421 [RP-002]Outlook.com rate limit on this connection.Your IP has sent more over a single connection than Outlook.com allows, a limit Microsoft ties to IP and domain reputation.
- 421 [RP-003]Outlook.com connection limit.Your IP has opened more connections to Outlook.com than it is allowed, a limit Microsoft links to IP and domain reputation.
- 421Temporary error or deferral.Yahoo or AOL refused your message for now with a 421, one of its two temporary error codes. Your server should keep it and retry; repeated deferrals point to reputation or complaints.
- 421Try again later.Xfinity turned the message away for now with a generic 421. Treat it as temporary and let your server retry: Xfinity uses it for several reasons it does not spell out.
- 421Reverse DNS lookup failed.Xfinity could not complete a reverse DNS lookup for your sending IP. It treats this as temporary, so your server should retry, but a broken DNS setup keeps it failing.
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