421-4.7.28 / 550-5.7.1: Unusual rate of unsolicited mail

GmailSMTP

The error usually looks like this:

421-4.7.28 Gmail has detected an unusual rate of unsolicited mail originating from your IP address. To protect our users from spam, mail sent from your IP address has been temporarily rate limited.

What it means

Gmail is rate-limiting or rejecting email from your IP address or domain because recipients have been marking it as spam.

Common causes

  • High spam complaint rates, sudden volume increases, or missing email authentication.

How to fix it

  1. Make sure SPF, DKIM and DMARC all pass, and include one-click unsubscribe in bulk mail.
  2. Check Google Postmaster Tools for your domain’s reputation, and reduce sending volume.

Tools to help diagnose it

Email blacklist checker

Check whether a mail server IP address or domain is listed on common email blocklists

SPF, DKIM, and DMARC checker

Check your domain’s email authentication DNS records, count SPF lookups and diagnose deliverability issues

Email header analyzer

Trace your email’s delivery path and check SPF, DKIM, and DMARC authentication results

A free tool provided by Auth-Email

Auth-Email makes your Gmail, Outlook, or other OAuth 2.0 email account available to any IMAP, POP3, or SMTP email client — no app passwords or special configuration needed.

Read the docsTry Auth-Email →

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