What do 4xx SMTP error codes mean? (Temporary failures)

4xx SMTP codes indicate temporary, transient conditions:

The signal:

"I cannot accept this message right now, but try again later." The failure is not permanent. Conditions may change.

Common 4xx codes:

421: Service not available, closing connection. 450: Mailbox unavailable (busy or temporarily blocked). 451: Local error in processing. 452: Insufficient storage. 454: Temporary authentication failure.

Sender response:

Queue the message for retry. Use appropriate backoff intervals. Eventually succeed or convert to permanent failure.

Why temporary matters:

Do not suppress addresses on 4xx alone. Give the system time to recover. Track patterns that may indicate deeper issues.

4xx is a polite "not right now." The door is not closed; just try again later.