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421 4.7.28: Gmail has detected an unusual rate of email

You sent more to Gmail, in a shorter time, than Gmail expects from you. It is holding the rest back for now.

Fix your sending setup Blocked or slowed down as possible spam

You sent too much, too fast, for Gmail.

  • Sent by: Gmail / Google Workspace
  • Temporary (soft bounce). The sending mail server retries on its own.

How Gmail words it (source):

Gmail has detected an unusual rate of email. To protect our users from spam, email has been temporarily rate limited.

Why it happens

  • A big mailing sent all at once.
  • Volume much higher than on your normal days.
  • The wording names what Gmail counted: your IP address, your IP range, your DKIM domain, your SPF domain, or a link in the message.

What to do

  1. Wait. The limit lifts on its own.
  2. Spread large sends out. Most email services have a throttle or "send over several hours" setting.
  3. If the text says "unsolicited", Gmail also thinks the mail is unwanted: trim the list to people who signed up and who open.

Don't confuse it with

550 5.7.28 is the permanent version: the IP is blocked, not slowed. 421 4.7.0 is about trust in the sender more than about speed.

Questions

Is this a bounce?

It is a deferral. The message is not lost yet; your mail system will try again.

What if the link in my email is the reason?

Then the domain you link to has a poor reputation at Gmail. Check where your links and tracking links point.

Got a different bounce?

Paste the whole message into the bounce code lookup. It reads the code and the words and tells you whose move it is.

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