When you select a message in the Inbox (or any folder) with the Reading Pane open, you can read the message without actually opening it. However, by default, Outlook marks that message as read. It won’t do so right away, and you can control how long Outlook will wait before marking the message as read. This behavior can be a bit of a nuisance if you just stop for a moment and don’t want the message marked as read. If you don’t notice the change, a message might fall through the cracks — especially if you rely heavily on Outlook’s visual status (bold when unread) to spot messages that may require your attention.
You can re-mark a message by right-clicking on it and choosing Mark As Unread from the context menu, but if this happens a lot, you should consider disabling this setting as follows:

  1. Choose Options from the Tools menu.
  2. Click the Other tab.
  3. In the Reading Pane section, click Reading Pane.
  1. Uncheck all of the options.
  2. Click OK twice.

Now, you can linger on a message as long as you like and Outlook won’t change the read setting until you actually open the message.