421 4.7.28: unsolicited 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.
- 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 SPF domain
What it means
This variant counts mail by the domain that passed SPF, which is the envelope sender (Return-Path) domain and not the From address. Gmail has seen an unusual rate of unsolicited mail from the domain named in the bounce and is temporarily rate limiting it. When you send through a provider, the SPF domain is often the provider's bounce domain, shared with its other customers, so their mail can count against you. A custom Return-Path on your own domain separates your reputation from theirs. Google's advice to pause for at least 10 minutes and then reconnect gradually applies here too.
Common causes
- Spam complaints or low engagement on mail sent with this envelope domain.
- A shared provider bounce domain whose other users send unwanted mail.
- A burst of volume under one envelope domain.
How to fix it
- 01Pause for at least 10 minutes, then resume on one connection and add connections one at a time.
- 02Set a custom Return-Path (bounce) domain on your own domain at your sending provider.
- 03Check spam rate in Postmaster Tools, and send only to engaged recipients until it falls.
- 04Spread large sends over the day instead of sending them all at once.
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 DKIM domain.Gmail is rate limiting mail signed with your DKIM domain, because it sees an unusual rate of unsolicited mail carrying that signature.
- 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