Presumably something pressed on the keyboard or mouse and lost the preview pane the contents of the letters in the Mail. By default, this area displays to the right of the message list. Shift version of the border region and list of letters to the right has not passed: on the other iMac it can be pushed up to the stop and push, and problem not even to catch hold of the mouse.
Switching to classic view (when view message at the bottom) does not help.
How to recover the missing viewport content?
Or maybe there's some way to reset the window?