How to Mark an Email as Read/Unread? How to Change the Status of the Message?

You can mark an email as read or unread in a few ways:

From the mail-view 3-dot menu: open the email, tap the 3-dot menu, and choose Mark as Unread.
From the Mail List (mobile): swipe left on the email and choose Mark Read / Mark Unread.
From the Mail List (desktop): right-click the email and select Mark as Read or Mark as Unread.

Mail View → 3-dot menu → Mark as Unread