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

What Gmail says

Gmail uses 421 4.7.28 for eight related situations. The key word in each is which thing was rate limited: IP address, IP netblock, DKIM domain, SPF domain, a URL domain in the body, or a repeated Message-ID.

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

2. 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.

3. Gmail has detected an unusual rate of unsolicited email originating from your IP Netblock ip-address. To protect our users from spam, email sent from your IP Netblock has been temporarily rate limited.

4. Gmail has detected an unusual rate of unsolicited email originating from your DKIM domain domain-name. To protect our users from spam, email sent from your domain has been temporarily rate limited.

5. Gmail has detected an unusual rate of unsolicited email originating from your SPF domain domain-name. To protect our users from spam, email sent from your domain has been temporarily rate limited.

6. Gmail has detected an unusual rate of unsolicited email containing one of your URL domains. To protect our users from spam, email with the URL has been temporarily rate limited.

7. Gmail has detected an unusual amount of unsolicited email originating from your IP address. To protect our users from spam, email sent from your IP address has been temporarily blocked.

8. Gmail has detected this sender exceeded the quota for sending messages that have the same Message-ID:. To best protect our users, the message has been temporarily rejected. For more information, go to Why has Gmail blocked my messages? (https://support.google.com/mail/answer/188131).

Texts 1 to 7 end with: "To review our bulk email senders guidelines, go to Email sender guidelines (https://support.google.com/mail/?p=UnsolicitedRateLimitError)."

Source: Google, Gmail SMTP errors and codes, section 421 4.7.28, fetched 2026-10-08.

What it means, in plain words

A 421 is a temporary SMTP code: the message was not accepted on this attempt. The text names the unit it limited, and that unit tells you what to change: an IP means the sending server, a netblock means your provider's range, a DKIM or SPF domain means your domain's reputation, a URL means a link in the content, a Message-ID means a sending tool reusing IDs.

Check your own side

• Read the exact text in the bounce and find the unit named.
• For texts 4 and 5, open Google Postmaster Tools for that domain and read the domain reputation and spam rate panels for the same dates.
• For text 8, read the Message-ID header on two consecutive messages from your tool. They must differ.

The fix

• Stop and queue.
• Lower the daily volume to the named unit and raise it over days, not hours.
• Texts 4 and 5: pause any list that produced complaints, remove hard bounces, and confirm every recipient opted in.
• Text 6: the link domain, not the sending domain, is the unit. Change or remove the link.
• Text 8: fix the sending tool so every message gets a unique Message-ID.

What this page covers: the authentication records the receiver checked and how to correct them. It does not predict where a message lands once accepted; authentication and inbox placement are separate questions.

FAQ

Will the limit lift on its own?

421 is temporary by definition. Gmail's text does not state a duration, and neither does this page.

Is 4.7.28 an authentication problem?

No. The texts name a rate or an amount, not SPF, DKIM or DMARC. Fix authentication separately if a 5.7.26, 5.7.27 or 5.7.30 appears in the same period.

Should I switch IPs to get around it?

Texts 4 and 5 name your domain, not an IP. Text 3 names your provider's IP range. A new IP does not change what those texts name.