SMTP Error Codes and Email Bounce Diagnosis

Choose the next check

1. Preserve the full response firstKeep the basic reply, exact enhanced code, provider text, time, and delivery context; this evidence is needed before choosing a retry, repair, or recipient-state action.
2. Interpret a 2.x.x result in contextA class-2 line reports that a server accepted the action it describes, but it can appear within a larger asynchronous event flow.
3. Triage a 4.x.x result as a current temporary conditionCheck the detailed code, timing, limits, routing, and provider guidance before a controlled retry with backoff.
4. Repair a 5.x.x result before another sendThe current attempt failed permanently; identify and correct the address, message, authentication, route, or policy condition instead of repeating it unchanged.

Common mistakes

• Acting on only the leading digit while discarding the exact enhanced code and provider text.
• Treating a 2.x.x response as proof of final inbox placement or treating every 5.x.x response as enough evidence to suppress a recipient.
• Retrying every 4.x.x response indefinitely instead of applying a bounded policy based on the detailed condition and current context.