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.
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