421 4.7.28 Gmail has detected an unusual rate of unsolicited email originating from your SPF domain [domain]
Sample bounce line
421 4.7.28 Gmail has detected an unusual rate of unsolicited email originating from your SPF domain example.com. To protect our users from spam, email sent from your domain has been temporarily rate limited. To review our bulk email senders guidelines, go to Email sender guidelines.
What it means
Gmail has temporarily rate limited email sent from your domain because it saw an unusual rate of unsolicited email from that SPF domain. It is a temporary (4xx) failure.
Why it happens
- Sending volume from the domain exceeds Gmail's limits or spikes suddenly.
- Recipients or Gmail's filters treat a high share of the mail as unsolicited (spam rate above Google's thresholds).
How to fix it
- Do not send email for at least 10 minutes.
- Identify the cause of the volume spike or spam complaints.
- After 10 minutes, send emails from a single connection.
- Start with low volume to engaged recipients and raise it gradually, avoiding sudden spikes.
- Keep the spam rate in Postmaster Tools below 0.10% and never reach 0.30%, and review Gmail's bulk email sender guidelines.
DNS record examples (replace example.com and the values with your own):
example.com. IN TXT "v=spf1 include:_spf.your-mail-provider.example ~all" ; one SPF record per domain; use your provider's real include value
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 email originating from your IP address [ip-address]