421 4.7.28: unsolicited mail from your IP netblock
Gmail is rate limiting a whole range of IP addresses that includes yours, because of the rate of unsolicited mail coming from that range.
- 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 unsolicited email originating from your IP Netblock
What it means
Here Gmail looks past the single IP to its netblock, the range of neighboring addresses it belongs to. Gmail has seen an unusual rate of unsolicited mail from that range and has rate limited it, so your mail can be caught even if your own IP behaves well. Hosting and cloud networks, where unrelated senders sit on adjacent addresses, are where this tends to happen. The reply is temporary. If the cause is your own traffic across several IPs in the range, Google's advice to pause for 10 minutes and reconnect gradually applies; if it is your neighbors, a different range or provider is the lasting fix.
Common causes
- Other senders in the same hosting range are sending spam.
- Your own traffic, spread across many IPs in one range, adds up to an unusual rate.
- A newly assigned range with no sending history.
How to fix it
- 01Pause sending to Gmail for at least 10 minutes, then resume on one connection and add connections one at a time.
- 02Check spam rate and IP reputation in Postmaster Tools to see whether your own mail is the cause.
- 03If your mail is clean, ask your provider for addresses in a different range, or move to a provider that keeps its ranges clean.
- 04Keep your own lists clean so complaints about your mail do not add to the range's problem.
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.
- 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