421 error: Service not available - try again later
The server is closing the connection for now: overloaded, rate limiting you or deferring on reputation. Temporary.
How to fix 421
- Retry later and reduce sending speed to that provider.
- If it persists for days, treat it as a reputation problem and check blacklists.
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 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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