421: too many mails to addresses that do not exist
GMX is turning your server away for a while because it sent too much mail to GMX addresses that do not exist. Clean the list before sending 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 Reject due to policy violations
What it means
Case r0902 means your server wrote to too many non-existent addresses at GMX, WEB.DE or mail.com, and GMX has temporarily rejected its mail as a result. Mail to unknown addresses is a common sign of an old or bought list, or of a spammer guessing addresses, so GMX treats a high rate of it as a policy violation. The reply is a 421, so delivery is retried. GMX asks you to remove hard bounced addresses from your list straight away and points bulk senders to its requirements, which warn that repeated mail to unknown or deactivated addresses can lead to suspension.
Common causes
- The mailing list holds many addresses that no longer exist or never did.
- Hard bounces from earlier sends were not removed from the list.
- Addresses were bought, rented or collected without confirmation.
How to fix it
- 01Remove every address that returned a hard bounce, starting with GMX case r1601 replies.
- 02Stop sending to addresses that have not engaged for a long time.
- 03Collect new addresses with double opt-in so typos and fake entries never reach the list.
- 04Read the GMX requirements for bulk senders before the next campaign.
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 r0902: 421 Reject due to policy violationspostmaster.gmx.net
Related codes
- 450Deferred for mail to unknown addresses.GMX deferred your mail with a 450 because your server has been sending to too many GMX addresses that do not exist.
- 550Mailbox does not exist or is disabled.The GMX, WEB.DE or mail.com address you wrote to does not exist, was disabled after a long period without use, or was blocked by the provider.
- 421IP blocked for mail to unknown addresses.IONOS has blocked your server IP from sending to its customers for now, because too much of your mail went to addresses that do not exist.
- 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