What 550 5.7.28 means
The message Gmail attaches to this code is “There is 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 5xx reply is permanent for that message, so the sending server gives up and returns a bounce. The enhanced status 5.7.28 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: The permanent form of 421 4.7.28: the IP's unsolicited-mail rate is too high. Stop the campaign, find the source (compromise, list, volume jump), warm.
Where warming fits
550 5.7.28 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
- 421 4.7.0 (Gmail): very low sending IP reputation
- 550 5.7.1 (Gmail): IP not authorized to send directly
- 550 5.7.1 (Gmail): very low sending domain reputation
- 421 4.7.0 (Gmail): very low reputation, deferred
- 421 4.7.28 (Gmail): unusual rate of unsolicited email
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 550 5.7.28
What does 550 5.7.28 mean?
Gmail returns 550 5.7.28 with the message "There is an unusual rate of unsolicited email originating from your IP address.": unusual rate, blocked. The receiver judged the sending domain or IP untrustworthy on its history. This is the class warming addresses.
Is 550 5.7.28 a hard bounce or a soft bounce?
A hard bounce for that message: the 550 reply is permanent, and the message is not retried. Whether the address should be removed depends on the cause: here the fix is on the sending side, not the list.
How do I fix 550 5.7.28?
The permanent form of 421 4.7.28: the IP's unsolicited-mail rate is too high. Stop the campaign, find the source (compromise, list, volume jump), warm. Gmail's own reference for the code is linked on this page.
Does email warmup fix 550 5.7.28?
It is part of the fix. 550 5.7.28 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.