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 toolbar, click the 3-dot menu 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.
To mark an open item, click the 3-dot menu in the item toolbar, and select the option.