Mail window views

Until very recently, when I selected "New Viewer WIndow" from the File menu, it would open a window to the right of the list of messages, that would show the entire text of the selected message. Very recently, when I select "New Viewer WIndow" from the File menu, it opens an entirely new window for Mail, that is identical to the one I already have, which is a 2-column layout with maiboxes in the left column, and a list of messages in the right column. Personally, I hate this layout and much prefer the 3-column format. I'm using Mail version 5.3 on an iMac. How do I fix this? I use OS 10.7.5