Outlook Icon on Taskbar or Notification area (System Tray)
Outlook
2016
The default settings minimize Outlook to the Taskbar, but you can make settings to minimize it to the Tray.
To enable this feature, follow these steps:
1. Right-click on the Outlook icon in the System Tray:

2. Select Hide When Minimized from the menu.
See also this tip in French: Comment réduire Outlook vers la barre d’état système.