First things first: OpenGoo is a really stunning software, I'm amazed at what you achieved with it! Go on with this amazing development speed!
Since the mail-system is only useful, if the mails are classified correctly, it should be very easy, to classify mails.
One way would be, to allow the user to drag and drop mails on a workspace. This should also work for multiple Mails at once. This would be such a time saver!
It would also be great, if one could classify multiple mails at once with the current dialog.
