SMTP reply classes (4xx and 5xx)
Also called: 4yz reply, 5yz reply, transient vs permanent failure
SMTP groups its numeric replies by first digit. A 4xx reply is a transient negative completion — the command failed but the condition is temporary and the client should try again. A 5xx reply is permanent, and the client should not repeat the same request.
In plain terms
The first digit tells you whether to retry. Four means try later; five means stop.
Why it matters for deliverability
Almost every deliverability decision starts here. Retrying a 5xx wastes reputation on a mailbox that will never accept the message, and treating a 4xx as final throws away addresses that were only temporarily unavailable.
What to do about it
- Read the enhanced status code before suppressing anything: 5xx covers recipient problems (5.1.x) but also sender, policy, authentication and content failures (5.7.x) that say nothing about the address.
- Suppress only recipient-invalid permanent failures. A 5.7.x rejection usually means fixing your authentication or your IP, not deleting a subscriber.
- Queue and retry 4xx responses on a backoff schedule rather than removing them.
- Track the two classes separately in reporting — a rise in 4xx usually means throttling, a rise in 5xx usually means list quality.
Source
RFC 5321 — Simple Mail Transfer Protocol
“4yz Transient Negative Completion reply ... the error condition is temporary, and the action may be requested again.”
Checked 2026-08-29
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Definitions reviewed 2026-08-29. Spotted something wrong? Tell us.
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