When dragging mail to a new folder in Mail, the item sticks to my cursor and won't let go.

I like to keep certain emails organized in folders. When dragging my mail today, the envelope icon on the cursor stuck to it, would not release to the selected folder and would not move the item. In the menu pull down, I could access an option under Message saying Move to (folder) again. Force Quitting helps, but after relaunching, the behavior recurs if I try to drag and drop. The envelope icon remains stuck to the cursor, even if I go to another application. It doesn't resolve until I Force Quit again.