421: reverse DNS lookup failed for now
GMX could not look up your IP address reverse DNS because of a temporary DNS error, so it deferred the connection. Your server's retry usually succeeds.
- 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 Requested action aborted: local error in processing
What it means
GMX only accepts mail when the reverse DNS of the sending IP can be read. In case r0604 the PTR query hit a temporary error, so GMX could not tell whether your IP has a valid name and refused the connection with 421. The reply is temporary on purpose: your server should try again later, and the problem may well be gone by then. GMX asks for patience first. If the error keeps appearing, the likely cause is the DNS infrastructure that serves your reverse zone, which is slow, overloaded or unreachable.
Common causes
- A temporary fault at the name servers that answer for your reverse zone.
- The reverse zone is served by name servers that time out or answer inconsistently.
- A short network problem between GMX and those name servers.
How to fix it
- 01Let your server retry; a single occurrence needs no action.
- 02If it repeats, query the PTR record of your IP from outside your network and time the answers.
- 03Ask the operator of your reverse zone (often your host or ISP) to check its name servers.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
Related codes
- 554No reverse DNS for your IP.GMX refused the connection because the IP address of your mail server has no PTR record, so it is not tied to any domain name in DNS.
- 451Temporary DNS error during the MX lookup.GMX could not complete the MX lookup for the sender or recipient domain because of a temporary DNS error, so it deferred the message.
- 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