A day or so ago I tried deleting about 4 messages off mail and then shut the app. A note came up and stated the messages could not be sent to the delete folder and then proceeded to close the window but the application remained as opened. I tried to quit the app from the dock but it wouldn't close it. I then tried shutting the system down and mail stopped it from closing. I have a Mac Book Pro with Mountain Lion 10.8.3 and tried to let the battery drain to let shut down that way to no avail. I've tried opening another window in Mail but it won't open a functional window (preview screen will not display highlighted message). How do I force a close or bring mail back to use it properly?