Exact response pattern
553 5.1.3 Mailbox name not allowedPermanentGenericRecipient
What it means
The recipient mailbox syntax or policy is invalid.
Likely causes
- Bad address syntax.
- Restricted local-part format.
Recommended response
- Correct the address.
- Validate before retrying.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.