Exact response pattern
551 5.1.6 User not local; please try forwarding pathPermanentGenericRouting
What it means
The user is not local to this server and the server will not forward automatically.
Likely causes
- Wrong destination host.
- Stale routing.
Recommended response
- Use the supplied forwarding path only if trusted.
- Correct MX or directory routing.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.