The Reading Pane is the area below the messages in your Inbox.
When you click on an email message, the message displays in a reading pane below the messages list. To adjust the size of the Reading pane, click then drag the divider bar up or down.
By default, viewing messages in the Reading Pane is on. You can turn it off on the toolbar View link.
In the Preferences Mail tab, you can configure whether you want messages you view in the Reading Pane to be marked as read or not marked as read
Mark it read immediately. As soon as you click a message in the Reading Pane it is marked as read.
Mark it read after "x " seconds. The default is 5 seconds. If you click on a message and view it for 5 seconds, the message is marked as read.
Do not mark it read. Messages you read in the Reading Pane are never marked as read.