SMTP 421
A transient reply meaning the service is not available and the server is closing the transmission channel. RFC 5321 notes it may be sent in response to any command if the server knows it must shut down.
In plain terms
The receiving server is hanging up. Not a rejection of the address — a rejection of the moment.
Why it matters for deliverability
421 is the classic throttling signal. A sudden run of them usually means you are sending too fast for that provider, or your sending IP's reputation has slipped enough for them to start closing connections early.
What to do about it
- Slow the sending rate to that provider rather than retrying at the same pace.
- Check whether the 421 text names a policy page or blocklist — many providers say why.
- Keep the address on the list; this is not evidence the mailbox is bad.
Source
RFC 5321 — Simple Mail Transfer Protocol
“421 <domain> Service not available, closing transmission channel (This may be a reply to any command if the service knows it must shut down)”
Checked 2026-08-29
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