Exact response pattern
550 5.1.6 Mailbox has moved, no forwarding addressPermanentGenericRecipient
What it means
The mailbox is no longer available and no forwarding destination exists.
Likely causes
- Account migration or deletion.
Recommended response
- Suppress the old address.
- Obtain an updated address through a legitimate source.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.