I loved Letterbox, a plug-in that allowed viewing Mail messages in a three-column vertical split window. After I upgraded to Leopard, I immediately tried to re-enable veritical split view. After much failed fussing with strings and MailVerticalSplitView, I decided to try editing the .nib files directly. What follows is a somewhat inelegant hack, but good enough for the time being (or until someone far better at using Interface Builder comes up with a fix): First, quit Mail, then back up, then go to this directory: /Applications/Mail.app/Contents/Resources/English.lproj. Open MessageViewerContents.nib with Xcode (Interface Builder). Click the divider, and in the Inspector, check the Vertical checkbox, and save. This will enable vertical mode, but cause some issues with the divider still being drawn. To fix this...

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