Possible to return to the 3 vertical Mail.app columns? 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 &amp; 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.
Hoshts is correct—I've made a recording showing what it will look like when you "resize" the invisible column: