Open your registry and find or create the key below.
Create a new DWORD value, or modify the existing value, called “MinToTray” and set it according to the value data below.
Exit your registry, you may need to restart or log out of Windows for the change to take effect.
User Key: [HKEY_CURRENT_USER\Software\Microsoft\Office\10.0\Outlook\Preferences]
Value Name: MinToTray
Data Type: REG_DWORD (DWORD Value)
Value Data: (1 = System Tray, 0 = Taskbar)