Fix "421 4.7.28": Gmail is rate-limiting your sending IP for unusual traffic

If your email is bouncing with 421 4.7.28, it means Gmail is rate-limiting your sending IP for unusual traffic. Here is exactly what causes it and how to fix it.

What 421 4.7.28 means

Gmail returns 4.7.28 when it sees a sudden spike or suspicious pattern from your IP. It is temporary but a clear warning that your reputation or ramp is too aggressive.

How to fix 421 4.7.28

• Cut volume and ramp gradually with warm-up.
• Segment to your most engaged recipients first.
• Fix authentication and complaints that trigger the throttle.

How Rainmail helps

Rainmail sends from your own domain on a dedicated, isolated IP with SPF, DKIM, DMARC, reverse-DNS and managed warm-up handled for you, plus an AI that watches your list health and deliverability. It also welcomes the senders Amazon SES, SendGrid and Mailchimp reject — from $8, cheaper per email than SES. Run the free 10-second checker or see plans.