For Outlook 2010:
1. Click the View tab.
2. In the Layout section, click on Reading Pane.
3.Select Options from the pop-up menu that appears.
4.With the Reading Pane window open, uncheck Mark items as read when viewed in the Reading Pane and click ok.
For Outlook 2007:
To stop Outlook from marking messages as read when viewed in the reading pane, do the following:
- Open Outlook
- Click on Tools on the top menu bar
- Select Options from the dropdown menu
- Select the Other tab
- Click the Reading Pane button
- Unselect "Mark items as read when viewed in the Reading Pane"
- Click OK
- Click Apply, then OK again