421 4.7.28 Our system has detected an unusual rate of unsolicited mail originating from your IP address
Gmail is rate limiting mail from your IP
Seen in Gmail, Google Workspace
Gmail temporarily rate limited your mail because the sending IP, or a domain in the message, is sending more unwanted mail than usual.
Common causes
- A sudden volume spike or a send to an old, unengaged list.
- High spam complaint rates from Gmail users.
- A shared IP or URL domain whose other users send spam.
How to fix it
- Slow down and let your server retry; do not hammer Gmail with immediate resends.
- Send only to opted-in, engaged recipients and add one-click unsubscribe.
- Make sure SPF, DKIM and DMARC pass, and watch your spam rate in Google Postmaster Tools.
Got a different message? Paste it into the error explainer.
Related errors
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- 550 5.7.1 Relaying denied / Message rejected · The receiving server refused the message on policy grounds
- 421 4.7.0 Try again later · Temporarily deferred by the receiving server
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- 550 5.7.1 Our system has detected that this message is likely unsolicited mail · Gmail blocked the message as spam
- 421 4.7.26 This mail has been rate limited because it is unauthenticated · Gmail deferred mail that passed neither SPF nor DKIM
- 421 4.7.27 Your email has been rate limited because SPF authentication didn't pass for this message · Gmail deferred or blocked mail that failed SPF
- 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
- 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