Apple Mail Preview Pane

The Correct Answer is, to grab the Dot in the middle of the bottom line bar of the window & slowly drag it up until you see the Preview pane below it. The PROBLEM is, being patient enough to effectively grab the bar's Dot! Until you move the Dot upwards, you haven't really gotten a hold of it. It is difficult to do, but I finally got it to work. My Sent Messages Mailbox was TOTALLY full, making it VERY difficult to grab & get it to move.