As you browse through your inbox, you’ll notice that unread mail appears in bold along with a blue indicator bar on the left. Outlook automatically marks messages as read and removes the blue unread indicator bar as they’re opened.
You can also toggle messages as read or unread manually.
Select a message in the list.
Click the Unread/Read button on the Home tab of the ribbon.
The message read/unread status is toggled.
You can also hover over a read message and click the unread indicator bar area, or right-click the message and select Mark as Unread.