I tried this option, and now sit with the folders on the left in Apple Mail on my Mac, then vertical preview pane (fine), and my full view message pane disappeared. No amount of 'handle pulling' from the right has restored the view.
I have checked preferences in Mail, and changed between the classic & other layouts to no avail. HELP.
I really want to return to the 3 vertical columns (folders, preview, and full view), in that order. Thanks.