What are common 4xx codes and their meanings? (e.g., 421, 451)

Common 4xx temporary failure codes:

421 Service not available: Server is shutting down or too busy. Retry after a brief delay.

450 Mailbox unavailable: Mailbox temporarily busy, locked, or blocked. Often used for greylisting.

451 Local error in processing: Server encountered an internal problem. Not your fault; try again.

452 Insufficient storage: Server or mailbox storage temporarily full. May clear as user deletes messages.

454 TLS not available: Temporary TLS/security issue. May resolve with retry.

Response to all 4xx codes:

Queue for retry. Use exponential backoff. Convert to permanent failure after retry exhaustion.

4xx codes invite patience. The receiving server is asking you to wait and try again.