I’m (was) having some difficulty with my network connection on my Mac. This is an unrelated error, but I found it digging in Console.
2007-02-22 11:10:26.479 Mail Loaded Letterbox bundle
2007-02-22 11:10:26.479 Mail Posing…
2007-02-22 11:10:26.821 Mail Bwahaha! I am a LetterboxExpandingSplitView, and I shall always be vertical!
Many thanks to Aaron Harnly the developer of Letterbox for such a humorous log