Try this work around.
Open Mail, not in Full screen mode. You need to be in classic lay-out.
< Command - , > to open preferences if you need to switch
Then from here <control-click> anywhere in the column area to make additions to the column
revert back to the new 3 column view. If you loose one of the columns grab one of the vertical sides and drag it to resize.
I had a strangely bizarre fix to this after fiddling around for a minute......
-In mail I hit: Command , (to get into options)
-Under the "Viewing" tab I clicked on "Use Classic Layout" and closed the options box.
My mail then reverted back to the old layout where sent messages were demonstrated accurately. After deciding that I didn't want to continue in the classic layout, I repeated the process above and unchecked the "Use Classic Layout" option and BANG, my sent messages remained in the correct format within the new mail layout!
I hope this works for others. These Mountain Lion Bugs are driving me mad.