Exact response pattern
501 5.1.7 Bad sender mailbox address syntaxPermanentGenericSender
What it means
The MAIL FROM address has invalid syntax.
Likely causes
- Malformed envelope sender.
Recommended response
- Correct the envelope sender formatting.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.