Outlook displays the read and unread emails in different colors and font weight. Normally the unread emails have blue colored subject line and once the email gets read, it turns to black.
However it is very easy to get the emails into read status even when you have just clicked it accidently just for a moment, so in the process you may miss some important emails. You can get through this problem by making a change in the Outlook settings in which you can control the amount of time needed to mark an email as Read when open.
1. Go to File > Options.
2. Click on Mail in the left pane and then click on Reading Pane… button located in the Outlook panes section.
3. In the dialog box box, you will see Mark item as read when selection changes in action by default, and so it is very easy to miss some emails. Check the box Mark items as read when viewed in the Reading Pane and then you can specify the time needed in seconds before marking an email as read. Default is 5 seconds and enough for most of us.
Click OK and that should do the trick!