By default, Outlook doesn't mark messages as read until a client has either switched to viewing another message in the reading pane or double-clicked to open the message. To modify this setting do the following steps:
- Click on File > Options and then click on the Mail section.
- Click the Reading Pane button to bring up options. Check the option that reads Mark items as read when viewed in the Reading Pane.
- You can also change the number of seconds (i.e. lower it to 1 second instead of the default 5). You'll notice that "Mark item as read when selection changes" can not be selected at the same time.
- Once you have finalized your settings, click OK.