Make your own Outlook! That's what I did. Well not really. But I am working on a new project (not TEO, not even tablet for that matter) that requires extensive Outlook UI customization and creating user interface prototypes in Outlook was becoming a major drag. So much code just to create a button or menu. And when you need to start getting hierarchical, forget it.
So I took some time writing a dummy Outlook client. It doesn't actually DO anything, but it looks almost identical to Outlook except for a few incorrect icons. But this one is much easier to customize in a WYSIWYG designer and allows me to turn out UI concepts much more quickly. I thought it was cool so I'd post a screen shot.