421 4.7.28: unsolicited mail from your DKIM domain
Gmail is rate limiting mail signed with your DKIM domain, because it sees an unusual rate of unsolicited mail carrying that signature.
- 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 DKIM domain
What it means
Gmail tracks reputation by the domain in the DKIM signature, the d= value, as well as by IP. The domain named in the bounce signs an unusual rate of mail that Gmail considers unsolicited, so mail signed with it is temporarily rate limited whichever IP it comes from. That is why moving to new IPs does not help. If the DKIM domain belongs to your sending provider rather than to you, other customers' mail counts toward the same limit, and signing with your own domain gives you a reputation of your own. Google's advice to pause for 10 minutes and reconnect gradually applies.
Common causes
- Spam complaints or low engagement on mail signed with this domain.
- A spike in volume from a campaign signed with the domain.
- A shared provider DKIM domain that other customers also use.
How to fix it
- 01Pause for at least 10 minutes, then resume on one connection and add connections one at a time.
- 02Check the domain's spam rate and reputation in Postmaster Tools.
- 03Sign with your own domain at your sending provider, not the provider's shared one.
- 04Cut volume to your engaged recipients until the limits stop.
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 4.7.28Unsolicited mail from your SPF domain.Gmail is rate limiting mail whose envelope domain passes SPF, because it sees an unusual rate of unsolicited mail from that domain.
- 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.
- 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