421 4.7.28: unusual rate of mail from your IP
Gmail is rate limiting mail from one of your sending IP addresses because the rate of mail it sees from that IP is unusual.
- Gmail
- Provider
- Temporary (4xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 421 4.7.28
- Provider
- Gmail and Google Workspace
- Temporary or permanent
- TemporaryThe sending server keeps the message and tries again later.
- Who has to act
- The sender. The fix is on the sending side: your domain, your server or your list.
- What the server says
421 4.7.28 Gmail has detected an unusual rate of email originating from your IP address
What it means
Gmail tracks how much mail arrives from each sending IP and how that compares with the IP's history. The IP named in the bounce has gone past what Gmail considers normal, so mail from it is temporarily rate limited. Messages queued on your server are retried, but retrying hard makes it worse, which is why Google's sender guidelines advise pausing for at least 10 minutes, then sending on one connection and adding connections one at a time. A new IP, or an IP that suddenly takes over the traffic of others, is a typical case.
Common causes
- A new or recently changed IP sending volume it has no history for.
- Traffic moved from several IPs onto one.
- A burst of messages from a campaign or a script.
How to fix it
- 01Stop sending to Gmail from the IP for at least 10 minutes.
- 02Resume on a single connection, and add connections one at a time once it works.
- 03Warm up new IPs by raising daily volume gradually.
- 04Check the IP's reputation and your spam rate in Postmaster Tools.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
- Gmail SMTP errors and codes (Google Workspace Admin Help)knowledge.workspace.google.com
- Email sender guidelines (Gmail Help)support.google.com
Related codes
- 421 4.7.28Unusual rate of mail.Gmail saw an unusual rate of mail from you and has temporarily rate limited it, to protect its users from possible spam.
- 421 4.7.28Unsolicited mail from your IP, temporarily blocked.Gmail has temporarily stopped accepting mail from your IP, which goes further than a rate limit, because of the amount of unsolicited mail it has seen from it.
- 550 5.7.28Unsolicited mail from your IP, blocked.Gmail has blocked mail from your IP because of an unusual rate of unsolicited mail from it. It is the permanent counterpart of the 421 4.7.28 limits.
- 421Service not available.The receiving server is closing the connection and will not take mail right now. The reply is temporary, so the sending server keeps the message and tries again later.
- 5.7.28Mail flood detected.The receiving system thinks the message is part of a flood of similar abusive messages, such as one caused by forged web form signups, and deferred or rejected it.
Further reading
All 108 Gmail codesEvery code in the wikiPaste your own bounce