421 4.7.28: unsolicited 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.
- 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 amount of unsolicited email originating from your IP address.
What it means
This is the strongest temporary form of 4.7.28. Instead of slowing mail from the IP, Gmail says it has temporarily blocked it, because of the amount of unsolicited mail it has seen from that address. The block can lift, but retrying through it achieves nothing, and keeping up the same traffic gives Gmail no reason to lift it. Google also documents a permanent block for the same problem, 550 5.7.28. Find out which stream on the IP draws complaints before sending more.
Common causes
- High spam complaint rates on mail from the IP.
- Sending to purchased, scraped or stale lists from that IP.
- A compromised account or server on the IP sending spam.
How to fix it
- 01Stop sending to Gmail from the IP, and check your logs for traffic you did not expect.
- 02Check IP reputation and spam rate in Postmaster Tools.
- 03Remove the stream that draws complaints, and clean your lists before resuming.
- 04When you resume, start on a single connection at low volume and raise it gradually.
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