You can mark items Read, Unread, Private, Complete, or Incomplete by right-clicking on the item in the Item List, or from the Preview Pane.
To mark an item in the Item List:
Right-click the item.
Select the option in the context menu.
To mark an item in the Preview Pane:
In the menu bar at the top, click to ellipsis button and select the option.
To mark a task Complete or Incomplete, click the Completed check box next to the date in the message at the top right.