421 4.7.28 / 4.7.0 error (Gmail): Unusual rate of mail - temporarily rate limited
Gmail saw a spike in mail from this IP or domain that looks unsolicited and is deferring part of it. Gmail retries are accepted later - this is throttling, not a permanent block.
How to fix 421 4.7.28 / 4.7.0
- Slow down: cut volume sharply and re-ramp gradually instead of retrying at full speed.
- Check the list - bounces and spam complaints are what trigger this; remove invalid and unengaged addresses.
- Spread sending across more mailboxes and domains rather than pushing one IP or domain harder.
Why this happens
Receivers cap how much mail they take from one source in a given window. Sending too fast, too much at once, or too many connections in parallel - especially from a new IP or domain - triggers a deferral or a limit.
How to keep it from coming back
Ramp volume gradually, cap per-mailbox and per-domain daily sends, spread a campaign over hours instead of minutes, and keep concurrent connections low for each provider.
Should I retry a 421 4.7.28 / 4.7.0 error?
Normally you don't have to do anything: a temporary error means the receiving server asked for a delay, and your mail server retries on its own for hours or days. Don't resend by hand on top of the queue. If the same deferral keeps coming back from one provider, treat it as a signal to slow down, not as bad luck.
Got the full bounce message?
The text after the code names the exact reason, and the same code can mean different things at different receivers. Paste the whole message into the decoder - it runs in your browser and nothing is sent.
Open the bounce decoderRelated error codes
If these deferrals keep returning, the cause is your volume or reputation. We can find it and sort it out with you.
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