Gmail 421 4.7.28
Rate limited: unusual rate of mail
Gmail 421 4.7.28 means: rate limited: unusual rate of mail. It is a temporary failure, so your server retries it. Gmail has seen an unusual rate of unsolicited mail from your IP, netblock, DKIM domain, SPF domain or a URL in the message. Slow down, stop mailing unengaged and unverified addresses, and check complaint rates in Postmaster Tools before ramping up again.
Gmail’s exact wording
Quoted from Google Workspace Admin Help: Gmail SMTP errors and codes, checked 2026-11-05. Placeholders are Gmail’s own.
Reply text
Gmail has detected an unusual rate of email. To protect our users from spam, email has been temporarily rate limited. To review our bulk email senders guidelines, go to Email sender guidelines.
Reply text
Gmail has detected an unusual rate of email originating from your IP address ip-address. To protect our users from spam, email sent from your IP address has been temporarily rate limited. To review our bulk email senders guidelines, go to Email sender guidelines.
Reply text
Gmail has detected an unusual rate of unsolicited email originating from your IP Netblock ip-address. To protect our users from spam, email sent from your IP Netblock has been temporarily rate limited. To review our bulk email senders guidelines, go to Email sender guidelines.
Reply text
Gmail has detected an unusual rate of unsolicited email originating from your DKIM domain domain-name. To protect our users from spam, email sent from your domain has been temporarily rate limited. To review our bulk email senders guidelines, go to Email sender guidelines.
Reply text
Gmail has detected an unusual rate of unsolicited email originating from your SPF domain domain-name. To protect our users from spam, email sent from your domain has been temporarily rate limited. To review our bulk email senders guidelines, go to Email sender guidelines.
Reply text
Gmail has detected an unusual rate of unsolicited email containing one of your URL domains. To protect our users from spam, email with the URL has been temporarily rate limited. To review our bulk email senders guidelines, go to Email sender guidelines.
Reply text
Gmail has detected an unusual amount of unsolicited email originating from your IP address. To protect our users from spam, email sent from your IP address has been temporarily blocked. To review our bulk email senders guidelines, go to Email sender guidelines.
Reply text
Gmail has detected this sender exceeded the quota for sending messages that have the same Message-ID:. To best protect our users, the message has been temporarily rejected. For more information, go to Why has Gmail blocked my messages?.
| Who sends it | Gmail and Google Workspace |
|---|---|
| Basic code | 421: <domain> Service not available, closing transmission channel (RFC) |
| Enhanced code | 4.7.28: Mail flood detected |
| Permanent or temporary | Temporary (4xx) |
| Verifying the list prevents it | Partly |
In practice
Gmail lists eight versions of this text, each naming what it rate-limited: your IP address, your IP netblock, your DKIM domain, your SPF domain, a URL domain in the message, an unusual amount of unsolicited mail, or too many messages with the same Message-ID. The named item tells you where Google sees the problem. A shared URL shortener or tracking domain can trigger the URL version even when your own domain is fine. Slow down, segment to engaged recipients, and check Postmaster Tools for spam rate.
What the sender should do
Gmail has seen an unusual rate of unsolicited mail from your IP, netblock, DKIM domain, SPF domain or a URL in the message. Slow down, stop mailing unengaged and unverified addresses, and check complaint rates in Postmaster Tools before ramping up again.
Would verifying the list have prevented Gmail 421 4.7.28?
Partly
The reply is not only about one address, but providers weigh how often a sender hits addresses that do not exist. Removing bad addresses before sending lowers that signal.
Common questions
- What does Gmail 421 4.7.28 mean?
- Gmail 421 4.7.28 means: rate limited: unusual rate of mail. It is a temporary failure, so your server retries it. Gmail has seen an unusual rate of unsolicited mail from your IP, netblock, DKIM domain, SPF domain or a URL in the message. Slow down, stop mailing unengaged and unverified addresses, and check complaint rates in Postmaster Tools before ramping up again.
- Should I retry after Gmail 421 4.7.28?
- Your sending server retries it automatically, because it is a temporary (4xx) failure. Investigate only if it keeps coming back over several hours.
- Would verifying my list have prevented Gmail 421 4.7.28?
- Partly. The reply is not only about one address, but providers weigh how often a sender hits addresses that do not exist. Removing bad addresses before sending lowers that signal.
The same code elsewhere
Sources
- Google Workspace Admin Help: Gmail SMTP errors and codes (checked 2026-11-05)
- IANA: SMTP Enhanced Status Codes registry (checked 2026-11-05)
- RFC 5321: Simple Mail Transfer Protocol, section 4.2.3 (reply codes) (checked 2026-11-05)
Registry and provider wording is quoted exactly; guidance is written by the SimpleVerifier Team.
SimpleVerifier Team · Last checked against the sources 2026-11-05