421 4.7.0 [TSS04] Messages from x.x.x.x temporarily deferred due to unexpected volume or user complaints
Yahoo is temporarily deferring your mail
Seen in Yahoo Mail, AOL
Yahoo is throttling mail from your IP because of a volume spike or complaints from its users; your server should retry later.
Common causes
- A sudden increase in volume, or a new IP that has not been warmed up.
- Recipients marked your mail as spam.
- Missing or failing SPF, DKIM or DMARC.
How to fix it
- Let your server retry with backoff and reduce the sending rate to Yahoo.
- Send only to engaged, opted-in recipients and honour unsubscribes quickly.
- Make sure SPF, DKIM and DMARC pass, and join the Yahoo complaint feedback loop.
Got a different message? Paste it into the error explainer.
Related errors
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- 421 4.7.26 This mail has been rate limited because it is unauthenticated · Gmail deferred mail that passed neither SPF nor DKIM
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- 550 5.7.25 The IP address sending this message does not have a PTR record · The sending IP has no reverse DNS
- 550 5.1.1 User unknown · The recipient address doesn't exist