421 4.7.28: unusual 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.
- 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.
What it means
This is the general form of Gmail's 4.7.28 rate limit, without saying whether it counted your IP, your DKIM domain or your SPF domain. Something about the volume or pattern of your mail looked unusual, so Gmail is accepting less of it for now. Google's sender guidelines say that when you exceed a quota, Gmail typically answers with 4.7.28, and that if the reply does not say which quota, you should assume all three are affected. Their advice is specific: send nothing for at least 10 minutes, then resume on a single connection and add connections one at a time.
Common causes
- A sharp rise in sending volume to Gmail.
- Many messages sent in a short burst instead of spread out.
- Mail that recipients report as spam, which lowers what Gmail will accept.
How to fix it
- 01Stop sending to Gmail for at least 10 minutes.
- 02Resume on a single connection; if it fails, wait another 10 minutes.
- 03Once a single connection works, add connections one at a time.
- 04Check spam rate and reputation in Postmaster Tools, and spread future sends more evenly.
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 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.
- 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