OS 10.4 - How can I get to an Inward mail item to delete it

Some time back I received a LONG Power Point Email message and hit Delete while it was downloading (A mistake). However the message heading still remains in the IN box even though I cannot open it. Now when I hit DELETE I get a spinning wheel that goes on forever but never deletes it. My problem is: I cannot quit Mail unless I Force Quit. Otherwise the Mail symbol in the bar is shaded dark blue. I suspect that long message is lurking somewhere in my system. Thanks.