Archived - Mac OS X 10.5: Mail shows no viewer window, can't be quit
Mail may open but show no message viewer window, and you may not be able to quit Mail. The Quit menu choice in the Mail menu may be dimmed ("grayed out"). This can happen in Mac OS X v10.5 through 10.5.6.
Download and install the Mac OS X v10.5.7 Update or later to resolve this issue.
To force quit Mail if necessary: From the Apple menu, choose Force Quit, select Mail in the Force Quit window's list of applications, then click "Force Quit".