By default, Outlook marks a message as read if you view it in the Reading Pane. If you'd rather retain the unread status, you can configure Outlook so it won't mark a message as read until you actually open it.
- Choose Options from the Tools menu.
- Click the Other tab.
- In the Reading Pane section, click Reading Pane.
- Uncheck all of the options.
- Click OK twice.