What 421 4.7.28 means
The message Gmail attaches to this code is “Gmail has detected an unusual rate of unsolicited email originating from your IP address.”, as written on its reference page. The first digit says how final it is: a 4xx reply is temporary, so the sending server queues the message and retries. The enhanced status 4.7.28 says what class of problem it is: reputation. The receiver judged the sending domain or IP untrustworthy on its history. This is the class warming addresses.
Why it happens
The receiver has a history for the sending domain and the IP behind it, and that history is bad enough to refuse mail outright. It is built from what the receiver's own users did with previous mail: marked it as spam, deleted it unread, never opened it, plus how much of it went to addresses that do not exist and how suddenly the volume changed.
What to do first: A volume spike or complaint spike from the IP (variants name the IP netblock, the DKIM domain, the SPF domain or a URL domain). Cut volume, check Postmaster Tools, keep warming.
Where warming fits
421 4.7.28 in this form is Gmail judging the sender's standing, and standing is built on engagement over time. Fix anything structural first (authentication, blacklists, list quality), cut the volume so the pattern stops, then rebuild the reputation the way it is earned: real engagement from real mailboxes over a ramp. That is what warming does, and a placement test shows when it has worked.
How to fix it, in order
- Check the domain in the reputation checker (blacklists, auth, age)
- Read the provider's own verdict: Postmaster Tools for Google, SNDS for Microsoft
- Cut the volume, then rebuild the reputation with warming
Related codes
- 550 5.7.511 (Microsoft): banned sender
- 550 5.7.606-649 (Microsoft): banned sending IP
- 550 5.7.28 (Gmail): unusual rate, blocked
- 421 4.7.0 (Gmail): very low sending IP reputation
- 550 5.7.1 (Gmail): IP not authorized to send directly
More on Gmail
- Google Workspace emails going to spam: the Postmaster-led diagnosis, dashboard by dashboard
- Gmail and Yahoo bulk sender requirements: the rules behind Google's authentication and spam-rate rejections
- Google Workspace email warmup: the ramp against Google's own sending limits
- Emails suddenly going to spam: the six changes behind a sudden drop, and how to confirm each
Questions about 421 4.7.28
What does 421 4.7.28 mean?
Gmail returns 421 4.7.28 with the message "Gmail has detected an unusual rate of unsolicited email originating from your IP address.": unusual rate of unsolicited email. The receiver judged the sending domain or IP untrustworthy on its history. This is the class warming addresses.
Is 421 4.7.28 a hard bounce or a soft bounce?
A soft bounce: the 421 reply is temporary and the sending server retries. If it persists for days, treat it as the permanent form and fix the cause.
How do I fix 421 4.7.28?
A volume spike or complaint spike from the IP (variants name the IP netblock, the DKIM domain, the SPF domain or a URL domain). Cut volume, check Postmaster Tools, keep warming. Gmail's own reference for the code is linked on this page.
Does email warmup fix 421 4.7.28?
It is part of the fix. 421 4.7.28 in this form is about the sender's reputation with Gmail, and reputation is built through engagement over time, which is what warming supplies. Fix authentication and list problems first, then warm.