OK, I found this suggestion in another thread. So far it has worked for my needs. Maybe some of you having this problem can use it too.
my current work-around for this annoying issue
go to control panel
go to internet options
go to programs tab
under opening internet explorer choose 'always in ie on the desktop'
keep ie full screen on the desktop but minimized to the task bar when not in use
then clicking a link in metro windows mail will open that link full screen in ie on the desktop.
That was the trick. Perfect. I hated that everytime I clicked a link in an Oultook messager that the screen split and opened Explorer in half. Got even worse last night when I clicked another link and now my screen was in thirds. Thanks for sending out the fix.