UI clarity issue
When you first open the app, on the right side there's no delineation between the toolbar and the message area, since there is no message selected. Even when a message is selected, there's no delineation between the 'Subject' area and the toolbar. The line that determines where one can click to drag the window and where one can't do so is invisible.
This certainly looks beautiful, but this is a problem for the UI because a user who isn't highly experienced with the Mac, or with the app won't know exactly where they should click if they want to drag the window to a different part of the screen. This problem would be less serious if there wasn't of late a lack of standardization for toolbar sizes in OS X these days. I would highly recommend putting in at least a faint line between the toolbar and the rest of the app to resolve this, at least as the default, if you're really attached to having it as an option.