In general, it is about how to prevent Outlook icon being hided in the task bar when minimizing. In the Task bar, click the Show hidden icon button to display the icons box. Find and right click the Outlook icon, uncheck Hide When Minimized option from the list.
From now on, when you minimizing the Outlook, the icon still be highlighted in the task bar and cause no confusion of closing for you anymore.
This affects Outlook 2007 (KB 3191898), Outlook 2010 (KB 3203467), Outlook 2013 (KB 3191938), and Outlook 2016 (KB 3191932).
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Update 12/24/15 am: An investigation by Ed Bott casts doubt on KB3xxxx being the problem, and points instead to an Office update that affected Office 2010, 2013, and 2016; the Windows 10 update still affects Edge, Outlook, and File Explorer.
Indeed the icon status showing in the task bar might cause confusion of this problem.
In this article, we will show you how to stop Outlook getting closed when minimized.