Exact response pattern
502 5.5.1 Command not implementedPermanentGenericProtocol
What it means
The server does not implement the requested SMTP command.
Likely causes
- Optional extension is unsupported.
Recommended response
- Use advertised EHLO capabilities only.
- Disable the unsupported extension in the client.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.