Exact response pattern
451 4.4.2 Bad connectionTemporaryGenericNetwork
What it means
The SMTP connection was unreliable or terminated unexpectedly.
Likely causes
- Network loss.
- Timeout.
- Middlebox reset.
Recommended response
- Retry.
- Review connection logs and timeouts.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.