In Outlook 2016, Outlook 2013 and Outlook 2010 by default the Home tab will be highlighted in Office Ribbon as shown below.
But if you want to keep the File tab as default highlighted tab then try hiding office Ribbon. This can be done by either pressing Ctrl + F1 or by clicking Minimize arrow to minimize the Office Ribbon.
Now the default tab highlighted will be the File tab. So if you have lot of operations using Options available as part of File tab then you can try this out.