How to Stop Mac Mail From Launching Automatically When Computer Is On

Boot the computer completely into Mac OS X.
Hold the 'Ctrl' key on the keyboard.
Click the Apple Mail icon in the Dock while still holding the 'Ctrl' key. A context menu appears. Release the 'Ctrl' key after clicking.
Move the mouse pointer over 'Options.' This presents a sub-context menu.
Click 'Open at Login' to uncheck it. The context menu automatically disappears after you click. The next time Mac OS X boots, the Apple Mail client will not start automatically.
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