When an email message is opened, the icon indicating its Unread status changes to Read. Users can mark a read email message as Unread and mark an unread message as Read. 

To Mark as Read / Mark as unread:

1. Right click on the email message. To select multiple emails use Shift key and click on the emails.
2. From the menu that pops up, select Mark as read or Mark as unread (indicated by the red oval) as per requirement.

3. The Read/Unread icon in the email message reflects the changed status (as shown in the image below)