Gmail 421 4.7.28: rate limited for unsolicited mail
421 4.7.28Rate limited for an unusual rate of unsolicited mailTemporary: the sender retriesFrom Gmail and Google Workspace
421-4.7.28 [203.0.113.30 15] 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. - gsmtp
What it means
A 4.7.28 deferral means Gmail has put a temporary throttle on the IP, or sometimes the DKIM domain, because recent mail from it looked like spam to Gmail users. The wording usually reads like the example and names either your IP address or your domain. The class digit 4 tells your server to keep the message queued and retry.
It is a reputation signal, not an authentication failure. Unlike 4.7.27 or 4.7.30, your records may be perfect. Gmail is reacting to volume patterns, spam reports, and how users engage. If the trend continues, the same traffic tends to start drawing 5.7.1 rejections instead.
Likely causes
- A sudden jump in volume, such as a campaign to an old list or a new IP sending at full speed with no warm-up.
- High user complaint rates; Google asks bulk senders to keep the spam rate shown in Postmaster Tools below 0.3 percent.
- A compromised account or web form on your server pumping out spam alongside your real mail.
- Shared IP neighbours on your ESP sending poor mail that drags the IP reputation down.
How to fix it
- Pause or slow nonessential bulk sends to Gmail. Let the queue retry legitimate transactional mail at a gentle rate.
- Check your outbound logs for abuse: sudden traffic from one account, unknown scripts, or a form relay. Stop it first.
- Register the domain in Google Postmaster Tools and watch IP reputation, domain reputation, and the user spam rate.
- Remove unengaged and unconfirmed addresses, add one-click unsubscribe (RFC 8058), and ramp volume up again slowly over days.
Checks that apply
- Blacklist Check Shows whether the same IP is listed on public DNSBLs, a hint that the spam is real and widespread.
- Google/Yahoo Compliance Verifies the bulk-sender items Gmail expects, including unsubscribe headers and DMARC.
- MailFlow Monitor Sends monitored test mail so you can see when Gmail stops deferring your traffic.
FAQ
- How long does a Gmail 4.7.28 throttle last?
- There is no fixed timer. It usually eases within hours to a few days once the volume and complaint pattern improve.
- Is there a form to request removal?
- Gmail offers a sender contact form for bulk senders, but there is no delisting portal for this throttle. Improving the traffic is what lifts it.