Letterbox is a plugin for Apple's Mail.app 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.
What’s new in version 0.24b9
Version 0.24b9:
This version of Letterbox is compatible with Mac OS X 10.6.7 and the build of Mail on new MacBook Pros.
Still incompatible with some Mail plugins that also use Sparkle for auto-updating.
The two-line combined From/Subject column doesn't play well with fonts other than Helvetica and Lucida Grande.
Changing the preview pane position with the menu item or keyboard shortcut does not visibly change the value in the preference pane.
Dev's site has changed, it's now: http://montivagant.com/software/letterbox/
And it states clearly:"I do not have a version of Letterbox for OS X 10.7 and 10.8. These versions of OS X support a 3-column view in Mail."
The Show/Hide function doesn't seem to work on 10.6 (though it does on 10.5), except for that, it's ok. ;-)
I have downloaded Letterbox for my wife's Macbook which runs OS 10.4. Having briefly enabled the Mail viewing pane to be to the right, it has now crashed Mail completely. Opening Mail brings up the top line menu but no Mail visible, so she has now lost everything. I have moved Letterbox to trash having attempted to uninstall 1st, with no effect. Help much appreciated.