How To Mark Emails as Read or Unread

1) To mark a single email unread

Both on iOS & macOS, right swipe that email and select the blue circle.

macOS,

iOS,

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

2) To mark all emails as unread / Read

macOS,

To mark email as read - Command + A (selecting all emails) > 2 finger tap in the thread pane > Select Read.

To mark email as unread - Command + A (selecting all emails) > 2 finger tap in the thread pane > Select Unread.

iOS,

To mark all emails as Read / Unread,

1. Tap the Edit 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