Exact response pattern
501 5.1.8 Bad sender system addressPermanentGenericSender
What it means
The sender domain or route is invalid.
Likely causes
- Invalid domain.
- Malformed address.
Recommended response
- Use a valid domain and address.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.