How to turn that off is what you want so here it is.

To turn off auto preview in ALL folders at once. In Outlook’s main screen,
you go to view, current view, define view, find the line that says “Messages
with autopreview,” choose modify and then look for the block that says
“other settings,” at the bottom of that screen uncheck the box that says
“show preview pane.” Then OK and Apply View your way out of the various
boxes.