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.
Should you remove the address?
Keep the address; fix the sending side
Keep the recipients, but stop sending to cold or unengaged parts of the list until the rate limit clears.
Soft bounce. Your sending server retries on its own. Investigate only if it keeps failing.
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.
Who sends it
Gmail and Google Workspace.
Gmail
Reply text Gmail has detected an unusual rate of unsolicited email originating from your DKIM domain <domain>. To protect our users from spam, email sent from your domain has been temporarily rate limited.
Gmail has versions naming your IP address, IP netblock, SPF domain or a URL domain in the message.
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.
- Is 421 4.7.28 a hard bounce or a soft bounce?
- 421 4.7.28 is a soft bounce. It is temporary, and your sending server retries it automatically. Your sending server retries on its own. Investigate only if it keeps failing.
- Should I remove the address after a 421 4.7.28 bounce?
- Keep the recipients, but stop sending to cold or unengaged parts of the list until the rate limit clears.
- Will verifying my list prevent 421 4.7.28?
- Only partly. Temporary deferrals are not about the address, but providers defer senders whose mail looks risky, and a high rate of hard bounces is one of the signals they use.
Clean the list before the next send.
A bounce report only tells you about the addresses you already mailed. The rest of the list has the same kinds of dead mailboxes in it, and each one becomes a hard bounce recorded against your domain the moment you send.
- A real SMTP mailbox check on every row, before anything is sent
- Dead and mistyped addresses split out, so they never reach your sending tool
- Catch-all, disposable and role addresses flagged alongside
Plans from $9 a month, cancel anytime, no contract. Sign in with Google and your first 100 email checks are free, no card. The free tools stay free either way.
Other 421 replies
Sources
- Source: Google Workspace Admin Help: Gmail SMTP errors and codes
- Source: RFC 3463: Enhanced Mail System Status Codes
Provider wording is quoted only where the provider publishes it.
Last reviewed 2026-10-22