How To Mark Emails as Read or Unread on iOS?

1) To mark a single email unread

On iOS, right-swipe that email and select the blue circle.

Note : An open email cannot be marked as Read / Unread.

2) To mark all emails as unread / Read

To mark all emails as Read / Unread,

1) Tap the More located on the upper right side

2) Select emails 

3) Tap select all (located at the upper left side)

4) Tap the blue circle (located on the lower right side) to mark emails as Read / Unread