Some systems do not minimize Outlook by default. If you want to "tray" Outlook you need to create this registry key and set the value of MinToTray to 1.
[HKEY_CURRENT_USER\Software\Microsoft\Office\10.0\Outlook\Preferences]
"MinToTray"=dword:00000001
To start the registry editor, from the Start menu choose the Run dialog and enter regedit. Navigate until you get to Preferences under Outlook. On the right side of the screen, right click and choose New | DWORD. Name it MinToTray and change the value to 1 (right click on it after naming it and choose Modify)
Close Outlook for the change to take effect. To remove the feature, delete or rename the key.