You can mark an individual E-Mail or a complete E-Mail thread as read or unread. Unread E-Mails will have a closed envelop icon in the sidebar.
How to mark an E-Mail as unread:
Select a read E-Mail in the sidebar.
Click on Mark unread in the display area.
In order to mark this E-Mail as read, click on Mark read.
Tip: You can also mark multiple E-Mails at once or all the E-Mails in an E-Mail thread as being read.