MailFixIt

Gmail temporary traffic and reputation error

Gmail 421 4.7.28: Gmail has detected an unusual rate of email

Gmail temporarily limited a sending stream. The full response identifies whether it concerns an IP, IP range, SPF domain, DKIM domain, URL domain, unsolicited mail, or repeated Message-ID.

Problem description

One Gmail response variant is:

421 4.7.28 Gmail has detected an unusual rate of email. To protect our users from spam, email has been temporarily rate limited.
Is this temporary or permanent?

SMTP 421 is temporary. Reduce pressure and keep retries slow while you investigate the identity named in the response.

Use the complete response

  1. Save the full response and timestamp.
  2. Note the IP, IP range, SPF domain, DKIM domain, URL domain, or Message-ID it names.
  3. Stop any unexpected campaign and secure compromised accounts, forms, or API keys.
  4. Remove invalid, unsubscribed, and unconsented recipients.
  5. Fix software that reuses one Message-ID for different messages.
  6. Resume legitimate mail gradually while watching Gmail Postmaster Tools, when available.

Do not rotate IPs or domains to bypass the limit. That hides the symptom and can spread the reputation problem.

Frequently asked questions

Is Gmail 421 4.7.28 temporary?

Yes. Keep controlled retries with backoff, but investigate repeated responses before restoring normal volume.

Does this code mean SPF failed?

No. A response may name an SPF domain, but 4.7.28 is a traffic or unsolicited-mail signal, not an SPF pass or fail result.

Related email diagnostics

Gmail 550 5.7.28: There is an unusual rate of unsolicited email originating from your IP addressResolve Gmail 550 5.7.28 when unusual unsolicited email from a sending IP is permanently blocked for the attempt. Yahoo: Message temporarily deferred (TS* errors)Resolve Yahoo Message temporarily deferred and TS* errors by slowing retries and finding the affected traffic or reputation signal. Outlook 451 4.7.500-699: Access denied, please try again laterResolve Microsoft 451 4.7.500-699 by keeping slow retries and investigating the changed IP volume or suspicious sending activity.

Primary sources: Gmail SMTP errors and codes, Google email sender guidelines.