Exact response pattern
451 4.4.0 Primary target IP address responded with failureTemporaryGenericRouting
What it means
The receiver could not complete routing to the next delivery component.
Likely causes
- Internal routing or MX failure.
- Gateway unavailable.
Recommended response
- Retry later.
- Check destination MX and route health if persistent.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.