Fix Gmail error 421 4.7.28: unusual rate of email

What the error says

Gmail documents this temporary error as: Gmail has detected an unusual rate of email. To protect our users from spam, email has been temporarily rate limited. Google's sender guidelines add that when you exceed your quota, Gmail typically rejects messages with SMTP error code 4.7.28.

Cause

Your domain or sending IP is sending faster than Gmail is comfortable with, or the traffic looks like spam. It is a 4xx code, so it is temporary: a well-behaved server queues and retries.

How to fix it

  1. Stop sending for at least 10 minutes, as Google's guidelines direct, then resume at a lower volume and ramp up gradually.
  2. Make sure your mail server retries deferred messages rather than dropping them.
  3. Confirm the domain is authenticated, since unauthenticated mail is throttled harder. Minimum records: 
    example.com.         TXT  "v=spf1 include:_spf.google.com ~all"
_dmarc.example.com.  TXT  "v=DMARC1; p=none; rua=mailto:dmarc@example.com"
  4. Enable DKIM signing with your own domain in your sending service.
  5. Review the list: remove bounced and inactive addresses and keep spam complaints low.

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Sources

More email delivery fixes