Gmail "421 4.7.28": unusual rate of unsolicited mail

What it means

Your server or email tool reports a temporary error from Gmail like:

421 4.7.28 Our system has detected an unusual rate of unsolicited mail originating from your IP address. To protect our users from spam, mail sent from your IP address has been temporarily rate limited.

The same code can mention your domain or a DKIM domain instead of the IP. Gmail is not refusing you for good: it is telling your server to slow down and try later. This is called a deferral.

Common causes:

• A sudden jump in volume, for example a big campaign from a new IP or domain.
• Many recipients marking your emails as spam.
• Sending to an old or bought list with many unknown addresses.
• A hacked account or website sending spam without your knowledge.

Why it matters

• Your emails arrive late, sometimes hours later.
• If the problem continues, Gmail moves from delays to sending your emails to spam or rejecting them with a 5xx error.
• Your server keeps retrying, which can fill its queue and delay all your other emails.

How to check

• Send a test email to the address on the home page /. Check that SPF, DKIM and DMARC pass and that your IP is not blacklisted.
• Check blacklists for your IP and domain at /blacklists.
• Use Google Postmaster Tools (postmaster.google.com) to see your domain's spam rate and reputation at Gmail.
• Look at your sending logs: did volume jump just before the errors started?

How to fix it

1. Slow down. Lower the sending speed to Gmail and spread big campaigns over several hours or days. Let your server retry deferred emails normally; do not resend them by hand.
2. Stop sending to people who do not want your emails. Remove unknown addresses, inactive subscribers and anyone who did not clearly sign up.
3. Make unsubscribing easy with a visible link and a one-click List-Unsubscribe header, so people unsubscribe instead of clicking "Report spam".
4. Keep complaints low. Google asks senders to stay under 0.3% spam complaints, ideally under 0.1%.
5. Check for abuse. If you did not send anything unusual, look for a hacked mailbox, contact form or website sending spam.
6. Warm up new IPs and domains by starting with small volumes to your most engaged readers and growing step by step.

Check that it is fixed

Watch your logs: 421 4.7.28 errors should become rare within a few days once volume and complaints are under control. Postmaster Tools should show the spam rate going down. Send a new test from the home page to confirm everything still passes.