Exact response pattern
550 5.4.0 Network or routing errorPermanentGenericRouting
What it means
A permanent routing error prevented delivery.
Likely causes
- No valid route.
- Broken connector.
Recommended response
- Verify MX and routing.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.