Exact response pattern
450 4.2.0 Mailbox busy or temporarily unavailableTemporaryGenericMailbox
What it means
The mailbox exists but is temporarily unable to receive the message.
Likely causes
- Mailbox lock or backend issue.
- Temporary quota handling.
Recommended response
- Keep queued and retry.
- Escalate only if the condition persists.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.