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SMTP error code · Gmail / Google Workspace

421 4.7.28: Gmail has detected an unusual rate of email

421 4.7.28 Temporary failure (4xx): the sender retries
By iBlockSpam Team Checked against sources 3 sources How we check our content
What it means

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 <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.

What 421 4.7.28 means

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.

Likely 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

If you sent the message

  1. Let queued mail retry; don't push it harder.
  2. Slow down and ramp volume up gradually on new IPs and domains.
  3. Check for compromised accounts or forms if you didn't plan a big send.
  4. Send only to people who asked for your mail, and make unsubscribing easy.
  5. Watch your spam rate and domain reputation in Google Postmaster Tools.

If you're the recipient or their admin

  1. No action needed at Gmail: the limit is on the sender's side.
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Frequently asked questions

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.

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.

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

  1. Gmail SMTP errors and codes (Google Workspace Admin Help)
  2. Email sender guidelines (Gmail Help)
  3. RFC 5321, section 4.2: SMTP replies (multiline reply format)

Provider messages are quoted as the provider publishes them. Servers can change their wording, so the bounce you received may differ slightly.

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