Exact response pattern
450 4.1.1 Mailbox unavailableTemporaryGenericMailbox
What it means
The mailbox could not be reached temporarily.
Likely causes
- Mailbox backend outage.
- Greylisting.
- Temporary account lock.
Recommended response
- Retry later.
- Do not suppress the address after one temporary failure.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.