421 4.7.28: unsolicited mail containing your link domain
Gmail is rate limiting messages that link to one of your domains, because that domain appears in an unusual rate of unsolicited mail.
- 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 containing one of your URL domains.
What it means
Gmail also tracks the domains that appear in links. One of the domains linked in your message shows up in an unusual rate of mail that Gmail considers unsolicited, so mail containing it is temporarily rate limited, whoever sends it. The domain can be your own, a link shortener, or a click-tracking domain shared by many customers of your sending provider. Since the limit follows the link, changing IPs or sender domains does little; removing or replacing the link is what changes the outcome.
Common causes
- A click-tracking or link-shortening domain shared with other senders.
- Your own domain being linked in spam, or in a campaign that people report.
- A sudden burst of messages that all contain the same link.
How to fix it
- 01Find the domain Gmail means by testing the message with and without each link.
- 02Use a click-tracking domain of your own instead of one shared by your provider.
- 03Replace public URL shorteners with full links.
- 04Pause for at least 10 minutes before resuming, and check the Safe Browsing status of your linked domains.
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 4.7.0Suspicious content or links.Gmail blocked this message because something in its text or its links looks like spam or abuse to Gmail's filters.
- 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