4.x.x — SMTP Temporary Failure Codes

Choose the next check

1. Capture the exact response and timingKeep the basic reply, enhanced code, provider text, destination, and attempt time; the generic class-4 label is not enough to select a retry policy.
2. Check recipient and mailbox availabilityA temporary mailbox or recipient condition may call for waiting or rate adjustment, not a permanent recipient-state change.
3. Check system, DNS, and routing conditionsWhen the response points to capacity, name resolution, connectivity, or routing, diagnose the relevant path before retrying at scale.
4. Check policy, authentication, and rate limitsProvider policy or anti-abuse deferrals need a specific repair or sending adjustment; more retries without that change can prolong the problem.

Common mistakes

• Retrying every class-4 response forever without a bounded backoff policy, response-specific limit, or escalation threshold.
• Adding a recipient to suppression based on a temporary code without checking the exact response and recipient history.
• Treating a provider rate-limit or authentication deferral as a generic timing issue instead of correcting the identified sender-side condition.