Exact response pattern
550 5.1.1 User unknownPermanentGenericRecipient
What it means
The destination reports that the mailbox does not exist.
Likely causes
- Typo.
- Mailbox deleted or never existed.
Recommended response
- Suppress the address immediately.
- Do not retry the same recipient.
- Review acquisition and validation practices.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.