I can't close my mail app
Option ⌥ click on Mail.app in the Dock and select Force Quit. (Force Quit is not the best practice for quitting an application as the app will exit "hard", but it is sometimes — rarely — necessary.)
I do not have the force quit option and I can't shut down my computer because of mail
Try Command-Option-Escape combination to open the Force Quit Applications window
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