Q: Mail won't open or quit so I cannot shut down my computer.Mail won't open or quit. computer won't shut down as a result

If you show Mail on the menu bar, click the apple at the left of the menu bar, and then Force Quit to make it quit.

Choose Force Quit from the Apple menu, select that application, and press the Force Quit button. If that doesn't work, save and close everything you can and use the Activity Monitor in the /Applications/Utilities/ folder to force quit the loginwindow process.

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If you are continuing to have problems, see http://support.apple.com/kb/HT3411