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Yahoo and AOL · temporary error

421: temporary 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.

Yahoo
Provider
Temporary (4xx)
Class
Sources checked
By , email deliverability consultant
Author

At a glance

Code
421
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 it means

Yahoo documents 421 and 451 as its temporary errors: something is blocking delivery at this moment, and a later attempt can still get through. The reasons it lists run from its own side, such as servers too busy to take the connection, which Yahoo says is generally brief, to yours: unusual traffic from your IP, content with traits of spam, complaints from Yahoo users, a name server that did not answer, or authentication that could not be checked. Yahoo also describes a group of deferrals labelled with codes that begin TS, caused by complaints, objectionable or spam-like content, a poor reputation for your IP or its subnet, or unusual traffic. It does not say which of the two codes each reason uses, or what each TS number means.

Common causes

  • Yahoo's mail servers were busy when your server connected.
  • Your sending IP, or the subnet around it, has a poor reputation at Yahoo or showed unusual traffic, such as a sudden rise in volume.
  • Recent mail from your server drew complaints from Yahoo users or looked like spam.
  • Your DNS or your authentication records could not be checked because of a temporary failure.

How to fix it

  1. 01Let your server retry on its normal schedule. Resending by hand only adds volume.
  2. 02If the error repeats, check that your SPF, DKIM and DMARC records resolve and pass, and that your name servers answer reliably.
  3. 03If a campaign or a new IP started just before, cut volume to Yahoo and AOL addresses back to a level that was delivering, then raise it gradually.
  4. 04Enroll your DKIM domain in Yahoo's Complaint Feedback Loop through Sender Hub, remove everyone who reports your mail as spam, and keep the spam rate below the 0.3% Yahoo sets.
  5. 05Read Yahoo's Sender Requirements and Recommendations and fix anything you do not meet, such as missing reverse DNS.
  6. 06If the same deferral continues for an extended period, submit a Sender Support Request on Sender Hub with the exact error and the diagnostic codes from your logs.

Sources

What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.

  1. 451Sender domain could not be resolved.Yahoo could not confirm that the domain in your envelope sender or From address exists. As a 451 it is temporary; Yahoo can also return the same problem as a permanent 554.YahooTemporary
  2. 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.StandardTemporary

421 at other providers

  1. 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.MicrosoftTemporary
  2. 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.MicrosoftTemporary
  3. 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.MicrosoftTemporary
  4. 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.ComcastTemporary
  5. 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.ComcastTemporary
  6. 421Temporary reject under an exceptional blocklist.GMX refused the connection from your server for now under one of the blocklists it applies in exceptional situations. Your server will retry; GMX asks you to stop the misuse first, then contact it.GMX, WEB.DETemporary

Further reading

All 7 Yahoo codesEvery code in the wikiPaste your own bounce

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