in Mojave, and other os have found this to be true, the preview window will not open:
solution:
restart, by resetting the pram: option, shift, p, r - hold down on start up.
then open mail
then jerk hard on the dot where preview window would open , mean very quick upward movement w mouse
iMac 27″ 5K, macOS 10.14
There should be no need to reset the PRAM.
All that is needed is to drag the edge at little more than you'd expect. Make sure that the cursor changes to a double arrow, and start dragging inward. At first it may seem that nothing is happening, but if you drag long enough the preview pane will suddenly pop open.
Note: the following screen capture has the preview on the right side, but it works just as well when the preview is at the bottom.
Apple Mail not showing the preview of the selected email in local mailboxes
