If Outlook, or any other program, are not appearing in the taskbar when minimized, check the following.

https://answers.microsoft.com/en-us/msoffice/forum/msoffice_outlook-mso_other-mso_2007/outlook-automatically-closes-when-minimized/4c3e492a-77f3-4dc7-b6d7-bc0bd5929165

It has likely minimized to the Notification Area of the Taskbar just to the left of the clock.  Right-click the Outlook icon in the system tray and uncheck "Hide When Minimized".