What 421 4.7.28 means
Gmail returns 421 4.7.28 when it sees an unusual rate of email, often specifically 'unsolicited' email, from one of your identifiers: the sending IP, the IP netblock, your DKIM domain, your SPF domain, or a domain linked in the message body. It then rate limits mail carrying that identifier.
The enhanced code 4.7.28 is registered with IANA as 'Mail flood detected'. It's temporary, but it's a reputation signal: Gmail has noticed volume that looks like a campaign recipients didn't ask for.
What the bounce looks like
The wording providers document for this code. Your bounce may add an IP address, a session ID or a help link after it.
421 4.7.28 Gmail has detected an unusual rate of email. To protect our users from spam, email has been temporarily rate limited.421 4.7.28 Gmail has detected an unusual rate of unsolicited email originating from your DKIM domain [domain]. To protect our users from spam, email sent from your domain has been temporarily rate limited.421 4.7.28 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.421 4.7.28 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.Why cold emailers see 421 4.7.28
- A sudden jump in daily volume from a domain or inbox, such as launching a new campaign at full speed on fresh domains.
- Many inboxes on the same domain all sending to Gmail at once, which adds up at the DKIM-domain level.
- A shared tracking or link domain that many senders use, so someone else's volume counts against yours ('one of your URL domains').
- Elevated spam complaints, so Gmail treats more of your mail as unsolicited.
- Re-sending the same Message-ID repeatedly; Gmail has a 4.7.28 variant for exceeding the quota of messages with the same Message-ID.
How to fix 421 4.7.28
- Identify which identifier is named in the text (IP, netblock, DKIM domain, SPF domain or URL domain). That's what you need to slow down.
- Reduce sends to Gmail recipients from that identifier for several days. Spread volume across the day instead of sending in bursts.
- If a URL domain is named, remove that link or switch to a custom tracking domain you own. Better yet, send the first email without links.
- Tighten targeting so fewer people report you. Complaints, not raw volume, are what turn a throttle into a block.
- Watch Google Postmaster Tools for spam rate and domain reputation, and ramp back up gradually once deferrals stop.
Will it retry on its own?
Don't suppress the recipient. Retries go through once the rate limit relaxes, but if you keep the same pace Gmail can escalate to 550 5.7.28, which is a permanent block of that mail.
Reading the code
The basic code 421 means "Service not available, closing connection (temporary)" under RFC 5321. In 4.7.28, the 4 is the class (persistent transient failure) and the 7 is the subject (security or policy). The detail number 28 is registered in RFC 3463 / the IANA registry as "Mail flood detected".
- Class 4
- Persistent transient failure
- A temporary condition delayed delivery. The message itself is valid, and sending again later may succeed.
- Subject 7
- Security or policy
- A security or policy decision: spam filtering, authentication, blocklists, permissions.
- Detail 4.7.28
- Mail flood detected
Check the rest of your setup
Paste the headers of a message that did get through into the email header analyzer to confirm SPF, DKIM and DMARC pass, and check your domain with the SPF, DKIM & DMARC checker. The deliverability checklist covers the rest of a healthy sending setup.
Got a different bounce? Paste it into the SMTP error code lookup to decode it.