Exact response pattern
501 5.5.4 Invalid argumentsPermanentGenericProtocol
What it means
The SMTP command includes unsupported or malformed arguments.
Likely causes
- Client implementation error.
Recommended response
- Capture the command and correct syntax.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.