click the "Test" link on the SMTP Server, the DNN reports the "Syntax error, command unrecognized. The server response was: authentication failed."

the SMTP user and password is no problem.  And then  I add the following smtp setting in the web.config to use the NTLM authentication:

    <system.net>
        <mailSettings>
            <smtp from="your@email.com">
                <network host="smtp.mail.com" password="XXXX" userName="your@email.com" defaultCredentials="false" />
            </smtp>
        </mailSettings>
    </system.net>

OK, the test has passed now! maybe it should be DNN bug for basic authentication??