What’s the difference between hard and soft failure codes?

• SMTP response codes indicate whether failures are permanent or temporary, guiding proper handling.
• Hard failures (5xx codes):
• Permanent rejection; don't retry this message to this recipient
• The server is definitively refusing delivery
• Common types:
• 550: User unknown, mailbox unavailable
• 551: User not local
• 552: Message too large
• 553: Mailbox name invalid
• 554: Transaction failed (policy rejection)
• Action: Suppress the address, investigate the cause, don't resend
• Soft failures (4xx codes):
• Temporary issue; retry is appropriate
• The server might accept the message later
• Common types:
• 421: Service unavailable
• 450: Mailbox temporarily unavailable
• 451: Local error, try again
• 452: Insufficient storage
• Action: Queue for retry with backoff, monitor for persistence
• Why the distinction matters:
• Retrying hard failures wastes resources and can look like spam
• Not retrying soft failures loses legitimate mail
• Proper classification drives appropriate suppression logic
• ESPs should handle this automatically, but understanding the logic helps troubleshoot unexpected behavior.

Edge cases: Some servers misuse codes (returning 5xx for temporary issues). The error text often clarifies actual meaning.