1. Click on the "File" tab in the top left corner of the Outlook window.
2. Click on "Options" in the left-hand menu.
3. In the Outlook Options window, click on "Advanced" in the left-hand menu.
4. Scroll down to the "Outlook panes" section and click on "Reading Pane".
5. In the Reading Pane settings, select "Mark item as read when viewed in the Reading Pane".
6. Uncheck the box next to "Mark items as read when viewed in the Reading Pane for (n) seconds".
7. Click "OK" to save the changes.