What does 504 mean?

SMTP 504 indicates an unimplemented parameter:

What it means:

The command is supported, but the specific parameter or option you used is not. The server cannot process this particular variation.

Common causes:

Unsupported authentication mechanism: AUTH method not available. Unsupported SMTP extension parameters: Options not implemented. Feature variations: Specific options disabled.

Typical message examples:

"504 Command parameter not implemented" "504 Authentication mechanism not supported" "504 Unrecognized authentication type"

Response:

Use an alternative parameter that the server supports. Check server capabilities via EHLO. Adjust configuration accordingly.

504 says "I support the command, but not that option." Find an alternative approach.