421 4.7.28: Gmail: unusual rate of unsolicited mail
Gmail is temporarily rate limiting your mail because it looks suspicious or unauthenticated.
Temporary failure (soft bounce)
What 421 4.7.28 means
Google's documentation lists this as a temporary rejection when it detects an unusual rate of unsolicited mail from your domain or IP, often tied to missing or failing authentication (SPF, DKIM, DMARC) or complaints.
Common causes
- Missing or failing SPF, DKIM or DMARC
- A spike in volume from a cold or new domain
- Recipients marking your mail as spam
- Sending to many invalid addresses
How to fix it
- Check your SPF, DKIM and DMARC setup
- Lower your volume and ramp up slowly
- Remove invalid addresses and unengaged contacts
- Check your Google Postmaster Tools reputation
What to do with the address
Keep the address, but clean the rest of the list and fix authentication first.
Check an address now with the free email checker, or look at a domain's mail setup with the email domain checker, MX lookup and SPF checker.
Related error codes
Frequently asked questions
- What does 421 4.7.28 mean?
- Gmail is temporarily rate limiting your mail because it looks suspicious or unauthenticated. Google's documentation lists this as a temporary rejection when it detects an unusual rate of unsolicited mail from your domain or IP, often tied to missing or failing authentication (SPF, DKIM, DMARC) or complaints.
- Is 421 4.7.28 a hard bounce or a soft bounce?
- Temporary failure (soft bounce). It may succeed on a later attempt.
- What should I do with the address after 421 4.7.28?
- Keep the address, but clean the rest of the list and fix authentication first.