421: connection ended by a temporary error
A temporary error occurred in the connection between your server and GMX, so GMX closed it. Another delivery attempt later should go through.
- 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 Service unavailable
What it means
Case r0304 is the plain temporary failure: something went wrong within the connection between your mail server and the GMX mail system, and GMX had to end it. The case page gives no cause beyond that and no action other than retrying, which is what a 421 asks for. Your server keeps the message and tries again on its own schedule. A one-off r0304 is not worth investigating. If it appears for every connection, look at what sits between your server and GMX, such as firewalls, TLS settings or an unstable link.
Common causes
- A short fault on the GMX side or on the network between the two servers.
- The connection was interrupted or timed out before it completed.
How to fix it
- 01Let your server retry later, as the reply intends.
- 02If every attempt fails the same way, check firewalls, TLS settings and network stability on your side.
- 03If it continues for a long time, contact GMX through its postmaster contact form with the full error line.
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 r0304: 421 Service unavailablepostmaster.gmx.net
Related codes
- 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.
- 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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