There are two ways to read a message: using the Reading pane, and opening the message into its own window.
When you click a message’s name in the List pane, the body of the message appears in the Reading pane below or to the right. Don’t forget that you can adjust the relative sizes of the List and Preview panes by dragging the border between them up or down.
To open a message into a window of its own, double-click its name in the List pane. An open message has its own Ribbon, along with Previous and Next buttons.
Once you’ve read a message, you can view the next one in the list either by pressing Ctrl+ (next message), by pressing Ctrl+U (next unread message), or by clicking its name ...