If you go to your user account settings / WebMail tab and check the "Mark messages sent to junk as read" does the trick. It's just an odd place for this setting as it would seem this is a web mail setting but it's not.
http://help.smartertools.com/SmarterMail/v15/Default.aspx?p=_DA&v=15.1.6005&lang=en-US&page=main/frmmysettings