To mark an email message as unread, position on the message and press Control + U. You can mark it as read by opening it again or by pressing Control + Enter.
Please note: if you have the Reading Pane displayed in Outlook, an email message will be marked as read automatically, as soon as you position on it. To switch off the Reading Pane, press Alt + V for the View Ribbon. Then press P, then N, to go to the Reading Pane menu. Finally press O for Off. You can also change the options for the Reading Pane in this menu.