Exact response pattern
550 5.3.0 Mail system errorPermanentGenericServer
What it means
The receiver reports a permanent system-level failure.
Likely causes
- Configuration or processing failure.
Recommended response
- Review the detailed diagnostic.
- Contact the recipient administrator if persistent.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.