# 421 4.7.28: Gmail has detected an unusual rate of email URL: https://iblockspam.com/smtp-errors/421-4-7-28 Last updated: 2026-10-09 Code: 421 4.7.28 (temporary failure, the sender retries) Provider: Gmail / Google Workspace Gmail is temporarily rate limiting mail from your IP address, network block, DKIM or SPF domain, or a link domain, because it saw an unusual rate of (often unsolicited) email. What the bounce says: ``` 421 4.7.28 Gmail has detected an unusual rate of email originating from your IP address . To protect our users from spam, email sent from your IP address has been temporarily rate limited. ``` Gmail's text (without its closing link), as listed in Google's SMTP errors reference. Other 4.7.28 variants name the IP netblock, DKIM domain, SPF domain or a URL domain instead. Gmail's replies can run over several lines; RFC 5321 says every line except the last starts with the code followed by a hyphen, so the same error can appear as 421-4.7.28. Google's SMTP errors reference lists eight 421 4.7.28 texts. Six say that Gmail detected an unusual rate of email and has temporarily rate limited it; one says email from your IP address has been temporarily blocked after an unusual amount of unsolicited mail, and one says a message was temporarily rejected because the sender exceeded the quota for messages with the same Message-ID. They differ in what is being limited: your IP address, your IP netblock, your DKIM domain, your SPF domain, a URL domain contained in the messages, or messages sharing one Message-ID. Google pairs most of them with a link to its email sender guidelines. A 421 is temporary, so your messages wait in the queue and are retried. The limit relaxes only when the pattern that triggered it stops. Typical triggers are a sudden jump in volume from a new IP or domain, a campaign that many recipients mark as spam, or a compromised account or website form sending spam through your server. Gmail's sender guidelines ask all senders to keep the spam rate reported in Postmaster Tools below 0.3%. If the text names a URL domain, the trigger is a link in your messages, which might be a shared link shortener or a tracking domain also used by spammers. If it names your DKIM or SPF domain, Gmail is limiting by domain reputation, so moving to a new IP won't help. ## Common causes - A sudden increase in volume from a new or previously quiet IP or domain. - Many recipients marking your messages as spam. - A compromised account, server or web form sending spam. - A link domain in your messages with a poor reputation. - Sending many messages that reuse the same Message-ID. ## How to fix it (sender) - Let queued mail retry; don't push it harder. - Slow down and ramp volume up gradually on new IPs and domains. - Check for compromised accounts or forms if you didn't plan a big send. - Send only to people who asked for your mail, and make unsubscribing easy. - Watch your spam rate and domain reputation in Google Postmaster Tools. ## How to fix it (recipient or admin) - No action needed at Gmail: the limit is on the sender's side. **Q: How long does a Gmail 421 4.7.28 rate limit last?** Google doesn't publish a fixed duration. The limit relaxes as the unusual pattern stops. Keep sending at a steady, lower rate and let your server retry queued messages. **Q: Will a new IP address fix 421 4.7.28?** Not if the text names your DKIM domain, SPF domain or a URL domain: those limits follow the domain, not the IP. Fix what triggered the spike instead. **Q: Does 421 4.7.28 mean my messages were lost?** No. A 421 is temporary. Your server keeps the messages and retries; they bounce only if the limit lasts longer than your server's queue lifetime. Sources: - Gmail SMTP errors and codes (Google Workspace Admin Help): https://knowledge.workspace.google.com/admin/support/troubleshooting/gmail-smtp-errors-and-codes - Email sender guidelines (Gmail Help): https://support.google.com/mail/answer/81126 - RFC 5321, section 4.2: SMTP replies (multiline reply format): https://www.rfc-editor.org/rfc/rfc5321#section-4.2 --- Source: iBlockSpam, https://iblockspam.com/smtp-errors/421-4-7-28 (this is a Markdown copy; the HTML page is canonical). Quote with attribution and a link. Site index for AI assistants: https://iblockspam.com/llms.txt