Exact response pattern
500 5.5.1 Command unrecognizedPermanentGenericProtocol
What it means
The server does not recognize the SMTP command or the command is malformed.
Likely causes
- Client syntax error.
- Unsupported command.
- Line corruption.
Recommended response
- Capture the SMTP transcript.
- Correct the command sequence or client configuration.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.