Gmail 421 4.7.28: “unusual rate of unsolicited email” (rate limited)

Short answer: Gmail is rate limiting mail from your IP, domain or links because it sees an unusual amount of unsolicited (spam-like) mail from them. It is temporary: mail is retried. Fix: cut spam complaints, authenticate properly and slow down, then watch your spam rate in Google Postmaster Tools.

The bounce looks like this
421-4.7.28 Gmail has detected an unusual rate of unsolicited email originating from your IP address. To protect our users from spam, mail sent from your IP address has been temporarily rate limited.
421-4.7.28 Gmail has detected an unusual rate of unsolicited email originating from your DKIM domain yourdomain.com …

What it means

Google's list has several 4.7.28 variants, one each for an IP address, an IP netblock, a DKIM domain, an SPF domain, a URL domain in the message, and repeated Message-IDs. The variant tells you what is being throttled. The cause is the same: recipients see too much of this mail as spam, or volume jumped suddenly. A 421 is temporary, but if it keeps up, mail starts going to spam or bouncing.

How to fix it

  1. Read which variant you got: IP (server or shared pool), DKIM or SPF domain (your domain's reputation), or URL domain (a link in the mail, often a shortener or tracking domain with a bad reputation).
  2. Check the domain below: SPF, DKIM and DMARC must pass, with DKIM signed by your own domain.
  3. Sign up for Google Postmaster Tools and keep the user-reported spam rate below 0.3% (Google's bulk-sender rule; under 0.1% is safer).
  4. Mail only people who opted in, add one-click unsubscribe to marketing mail, and drop addresses that never open.
  5. Spread big sends out and ramp up volume on new IPs or domains gradually.
  6. Check for spam leaving your server: a compromised account or website form is a common cause of a sudden 4.7.28.

Check your domain now

After the fix, send a test email to see SPF, DKIM and DMARC pass on a real message, then keep an eye on it: free DMARC monitoring shows every service that sends mail as your domain and which ones fail, in plain English.

Rather not touch DNS? I set up SPF, DKIM and DMARC without breaking your other senders, with a before/after report.

Get it fixed on Fiverr

Prefer to do it yourself? Get the step-by-step Fix Kit ($15) – 28-page guide, checklist, 20+ providers.

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