Letterbox is a plugin for Apple’s Mail that takes advantage of your widescreen monitor. It rearranges the interface into three vertical columns, so the message pane is to the right of the message list, rather than below. Especially on widescreen Macs, this layout makes better use of your available screen real estate, and gives emails a more pleasing paper-like shape. You?ll probably need at least 1280 and perhaps 1440 horizontal pixels for this layout to work well for you.