1) To mark a single email unread
Both on iOS & macOS, 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 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.
To mark all emails as Read / Unread,
- Tap the Edit located on the upper right side
- Select emails
- Tap select all (located at the upper left side)
- Tap the blue circle (located on the lower right side) to mark emails as Read / Unread