Hey, I just had an idea for the visual clue about moving a message.
The Folders list has a header bar 'Folders'. What if we added a header bar to the message view that you could drag? The left edge of the message header bar could also have a little document/message icon in it. I don't know if this is enough visual clue to the user that they can/should drag the header bar to move the message.
Any thoughts? I'm just brainstorming here.
Jeff