Exact response pattern
550 5.2.0 Mailbox unavailablePermanentGenericMailbox
What it means
The mailbox is unavailable for a reason not more specifically classified.
Likely causes
- Disabled account.
- Mailbox policy.
Recommended response
- Use the accompanying text to decide suppression.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.