To If you wish to use Internet Explorer, do the following. Main Messenger Window > Plus > Preferences > Plugins > StuffPlug-NG > Configure > Main > Uncheck the circle next to Open all links in default browser.
If you wish to use another custom browser to view all incoming mail do the following if you only use Internet Explorer for the internet, ignore this; (anything that isn't internet explorer, like Mozilla Firefox for example do the following. Start > Control Panel > Add\Remove Programs > Set program access and defaults > Custom). Set the browser you wish to use as default browser default web browser. Click OK.
This should fix both problems. Hope it helps
. If you need more help, please ask.