Outlook provides an option for marking an already read email items as unread. You can do this for easy identification of email messages and if you want to revisit them at a later time. You can mark an email message as unread by either a Keyboard shortcut or using the menu option. Listed below are the different ways to mark an email message as Unread in Outlook 2013 and Outlook 2010
Select the email message which you can mark as unread as press Ctrl + Q.
Navigate to Home menu and select the email message and click on the Unread/read menu available as part of Tags option
These are toggle switches you can also use the above options for marking a new email message as a read item. Another alternative way is to use o right click context menu and pick Mark as Unread from the menu list.