Right click on a blank area of the tool bar, and a menu will open. Point to Customize Command Bar, then click Add or Remove Commands. Find the Mail command button on the left and add it to your Tool Bar.
I like most of the new features of IE7; the speed and the way the toolbars are arranged. I think someone forgot to remember to put back the MAIL button on the browser to get to your default mail program. I can't get to my default mail (outlook express) without going to my start menu.

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