421: too many messages in a short time
GMX received too many messages from your system in a short period and blocked it for a while. GMX can raise the limit on request.
- 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 Reject due to policy restrictions
What it means
Cases r0401 and r0404 are a volume limit. Your system delivered more messages to GMX in a short period than it accepts, so GMX blocked it temporarily with a 421 and your server will retry. Unlike the GMX limits on connections, transactions and recipients, this one can be raised: the case page invites you to ask for a higher limit through its contact form, and to tell GMX about any additional IP addresses or ranges (in CIDR notation) you send from, so they can be considered together. Until then, spreading the volume over a longer period keeps you under the limit.
Common causes
- A campaign or batch sent a large volume to GMX addresses at once.
- A new sending IP started at full volume without a gradual ramp.
- Several of your servers send to GMX and their traffic adds up.
How to fix it
- 01Spread sending to GMX, WEB.DE and mail.com addresses over a longer period.
- 02Ramp up new IPs gradually rather than at full volume.
- 03Ask GMX for a higher limit through the form on the case page, listing all the IPs and ranges you send from.
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 r0401: 421 Reject due to policy restrictionspostmaster.gmx.net
- GMX Postmaster case r0404: 421 Reject due to policy restrictionspostmaster.gmx.net
Related codes
- 450Too many messages in a short time (450).GMX blocked your system for a while with a 450 because it received too many messages from it in a short period. The limit can be raised on request.
- 451Far more mail than expected.GMX received far more mail than it expected from your system, your DKIM domain or your network, and deferred the rest for now. A higher limit can be requested.
- 421Rate limit reached.Your server sent too many emails to IONOS in too short a time, so IONOS deferred the rest. Send fewer at once and try again later.
- 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.
Further reading
All 57 GMX, WEB.DE codesEvery code in the wikiPaste your own bounce