Problem with Mail

Are you sure you didn't just drag the divider bar all the way to the bottom? If so, just grab it on the bottom of the window and drag it up. If you see a dot in the middle of the bottom of the window, that's it.

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At the bottom of your Mail/inbox window, is there a think gray bar with a dot on the middle, like this?


If so, click it, hold the mouse button down and drag the bar up.