Exact response pattern
504 5.5.4 Command parameter not implementedPermanentGenericProtocol
What it means
The command is supported but a requested parameter or mechanism is not.
Likely causes
- Unsupported AUTH mechanism.
- Unsupported extension parameter.
Recommended response
- Choose one of the mechanisms advertised in EHLO.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.