421 4.7.28 Gmail bounce: rate limited
What 421 4.7.28 means
Gmail slowed mail from your sender because the volume spiked or looks unsolicited. The rate limit is temporary.
Message text
Gmail has detected an unusual rate of email. To protect our users from spam, email has been temporarily rate limited.
Common causes of 421 4.7.28
- Sudden spike in sending volume
- Recipients not engaging or marking mail as spam
- Sending to old or unverified lists
How to fix 421 4.7.28
- Slow down and ramp volume up gradually.
- Remove bounced and inactive addresses from the list.
- Check your sending IP and domain with blocklist check.
Check your domain
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How to read 421 4.7.28
The first part, 421, is the SMTP reply code. Service not available, closing transmission channel: the server is shutting down or refusing the connection for now. Replies that start with 4 are temporary and replies that start with 5 are permanent (RFC 5321 §4.2.1).
The second part, 4.7.28, is an enhanced status code (RFC 3463 §2) written class.subject.detail. Class 4 is a Persistent transient failure (4xx): the command was not accepted, but the condition may clear, so the sender should try again later (RFC 5321 §4.2.1; RFC 3463 §2).. Subject 7 is security or policy: authentication, authorization or policy refused the message. The detail number refines the subject, and providers may define details of their own, which is why one code can read differently at Gmail, Microsoft and Yahoo.
Sending servers should retry a deferred message. RFC 5321 §4.5.4.1 advises waiting at least 30 minutes between attempts and trying for at least 4 to 5 days before giving up and returning a bounce. If the deferral keeps repeating, fix the cause rather than waiting.
Official source: knowledge.workspace.google.com. Last verified 2026-10-20.
Related guides
- How to set up a DMARC record, from p=none to p=reject
- How to create an SPF record (syntax and examples)
- How DKIM works and how to publish a DKIM record
Related codes
FAQ
What does 421 4.7.28 mean?
Gmail slowed mail from your sender because the volume spiked or looks unsolicited. The rate limit is temporary. This is a temporary failure.
How do I fix 421 4.7.28?
Slow down and ramp volume up gradually. Remove bounced and inactive addresses from the list. Check your sending IP and domain with blocklist check.
Is 421 4.7.28 permanent, and should I retry?
421 4.7.28 is a temporary failure. Your mail server normally retries by itself; if it keeps repeating, fix the cause.