Exact response pattern
550 5.6.3 Conversion required but not supportedPermanentGenericContent
What it means
The receiver cannot perform required content conversion.
Likely causes
- Unsupported legacy format.
Recommended response
- Use standard MIME and transfer encoding.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.