Send emails with external server

I am trying to configure webmin to send emails using an external server (Webmin -> WebmIn Configuration -> Sending Email).

The external server requires SSL. I enter all the required details, save and then try and send a test email. Each time I just get the error message "sending failed : SMTP command failed". I check all the mail logs and can't find any additional information about the cause of the failure. Do you know where this information would be logged? I have no idea where the process is failing.

Cheers