421 4.7.28 Gmail has detected an unusual rate of email originating from your IP address [ip-address]
Sample bounce line
421 4.7.28 Gmail has detected an unusual rate of email originating from your IP address 203.0.113.10. To protect our users from spam, email sent from your IP address has been temporarily rate limited. To review our bulk email senders guidelines, go to Email sender guidelines.
What it means
Gmail is temporarily rate limiting email sent from your IP address because the sending rate looks unusual. It is a temporary (4xx) failure.
Why it happens
- Sending volume from the IP exceeds what Gmail accepts, or there was a sudden spike in volume (Google: exceeding your quota typically produces 4.7.28; immediately doubling previous volume can cause rate limiting).
- Spam rate or reputation problems for the sender.
How to fix it
- Do not send email for at least 10 minutes.
- Identify the cause of the spike or complaints.
- After 10 minutes, send emails from a single connection.
- Start with low volume to engaged recipients and raise it gradually instead of sudden spikes.
- Keep the spam rate in Postmaster Tools below 0.10% and never reach 0.30%.
Check your domain now
Many of these errors come from missing or broken SPF, DKIM, DMARC or reverse DNS records. Enter your domain to run the free check.
Related errors
- 421 4.7.28 Gmail has detected an unusual rate of unsolicited email originating from your SPF domain [domain]
- 550 5.7.1 This message is likely unsolicited email