421 4.7.28 Gmail: unusual rate of email, rate limited
Quick answer
Gmail is temporarily rate limiting the sender. The reply reads "Gmail has detected an unusual rate of email", and its variants name the source: the sending IP, IP netblock, DKIM domain, SPF domain or a domain in the links. The message was deferred, not rejected, and the recipient is not the problem.
Recommended action: Retry later
Common causes
- A sudden jump in volume from a new or recently quiet IP or domain.
- High spam complaint rates from Gmail users on recent campaigns.
- Sending to old or purchased lists with many invalid addresses and spam traps.
What to do
Slow down and let your mail server retry. Then fix the cause: verify the list to remove invalid and risky addresses, cut volume to unengaged recipients, and watch your spam rate in Google Postmaster Tools. Google expects bulk senders to stay below 0.3%.
Provider note: RocketVerifier recognises 4.7.28 as a block on the sender: during verification the address comes back unknown with the blocked sub-status, free, and the probing IP rests.
How to read 421 4.7.28
421 is the basic SMTP reply defined in RFC 5321. 4.7.28 is the enhanced status code from RFC 3463, which reads as class, subject and detail:
- Class
- 4Temporary failure
- Subject
- .7Security or policy
- Detail
- .28Gmail: unusual rate of email, rate limited
Further reading
FAQ
Frequently asked questions
How long does a 421 4.7.28 deferral last?
Google does not publish a fixed time. Deferrals lift once the volume and complaint pattern returns to normal, which usually means hours to days of lower, cleaner sending.
Is 421 4.7.28 the same as being blocklisted?
No. It is Gmail's own rate limit, not a public blocklist listing. Check public blocklists separately with our Blacklist Checker.
Stop hard bounces before they start
Upload a list or call the API, and send only to addresses that can receive mail. Unknown results are always free.
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