When you select a message in the message list, you can see the message in the Reading Pane and access many different types of file attachments. If you don't want to preview messages without opening them, you can turn the Reading Pane off.
To turn off the Reading Pane, click View > Reading Pane > Off.
This closes the Reading Pane only for the folder you’re in. To change other mail folders, in the folder pane, click the folder, and then repeat the previous steps.
You can also turn off the Reading Pane quickly for multiple folders. For example, if you turned off the Reading Pane in your Inbox, while still in your Inbox, go to View > Change View > Apply Current View to Other Mail Folders and then select folders in the Apply View dialog box.
If you later want to turn on the Reading Pane, click View > Reading Pane, and then Right or Bottom.