A lot of comments have been made about making the window size adjustable for the preview window on the bottom. I have not seen anyone comment to have the preview window on the right side of the message threads instead of below them. I am not sure what kind of screens are the most common, but i have a 24" at home and a 27" at work. There is more than enough screen real estate to have the preview window on the right. For text based emails, it would simply mean the message would show up on more lines as there is less available width. For HTML pages, most of the time they are not full width anyhow and would look better in a narrower window.
Take a look at how outlook.com has their layout done. Folders on the far left, then a 2-3 line message thread view, then the preview on the far right. IF the preview window is collapsed, then the multi-line message headers becomes 1 line.
Not sure what others would think of this.