Temporary — will retry
SMTP 421 4.7.28
unusual rate of mail from your IP (Gmail)
SMTP 421 4.7.28 means Gmail has seen an unusual rate of mail from your IP or domain and is deferring it to protect its users. Google's message reads "Gmail has detected an unusual rate of email originating from your IP address." It is temporary, but it is a reputation warning.
How it appears
421 4.7.28 Gmail has detected an unusual rate of email originating from your IP address. To protect our users from spam, mail sent from your IP address has been temporarily rate limited.
What causes it
- A sudden jump in volume from an IP or domain with little sending history.
- High spam complaint or bounce rates on recent sends.
- A compromised account or server sending spam from your IP.
How to fix it
In order — try the first one first.
- Slow down. Lower the sending rate to Gmail and let the deferred mail retry.
- Warm new IPs and domains gradually rather than starting at full volume.
- Check Google Postmaster Tools for spam rate and domain reputation.
- Clean the list before the next send so bounces and spam traps do not push reputation down further.
Common questions
- What does SMTP 421 4.7.28 mean?
- SMTP 421 4.7.28 means Gmail has seen an unusual rate of mail from your IP or domain and is deferring it to protect its users. Google's message reads "Gmail has detected an unusual rate of email originating from your IP address." It is temporary, but it is a reputation warning.
- Should I retry a 421 4.7.28 error?
- Yes, and your sending server does it automatically. 421 is a temporary deferral, and legitimate mail usually arrives on a retry within minutes or hours. Only investigate if it persists across many retries.
- How do I fix SMTP 421 4.7.28?
- Slow down. Lower the sending rate to Gmail and let the deferred mail retry.
Other 421 replies
Last reviewed 2026-09-11