If you are like me and have a habit of quickly glancing at most emails, but want to read them again later, this tip can be especially useful. The default setting for marking emails as Read in Outlook is 5 seconds. That means if the email is open in your reading window for 5 seconds, it’s marked as read. If you want to skim and be able to come back and see those marked as unread later on, you can increase the time the email needs to be open to marked as Read.

First, go to File, then Options, then Advanced >  Next, click on Reading Pane > Adjust the settings in the Reading Pan pop up to as much time as you would like.