Outlook’s default mail configuration displays three windows: the Navigation pane, the current folder, and the Reading pane. If you spend most of your time in the Inbox or in a few personal folders, reading and responding to mail, you seldom use the Navigation pane. By hiding that pane, you can free up a third of your screen to make more room for messages.
To hide the Navigation pane, press [Alt]+[F1]. To get it back, just press the same keys — it’s a toggle shortcut.