Folder behaviour like Windows with Improvements
Allow me to look into a folder when I drag the email to it. So I can see whether it is the one I expected the target subfolder in. Without dropping it I can drag the mail to another folder or to one of the subfolders that show up in the first folder to find my target folder.
Further improvement if the folder I point on for a little more time closes when it I realize I missed the one I wanted. Then I can look into the next folder still dragging without dropping the mail to the wrong folder or to drop it back to the origin.