Exact response pattern
501 5.5.2 Syntax error in parameters or argumentsPermanentGenericProtocol
What it means
The command is understood but one or more parameters are invalid.
Likely causes
- Malformed address.
- Invalid command argument.
Recommended response
- Correct the SMTP syntax.
- Validate envelope addresses and client formatting.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.