.ReadMsgBody{width: 100%;}
.ExternalClass{width: 100%;}

http://www.frontoftheweb.com/centre-aligning-html-emails-in-hotmail

A small tip I picked up yesterday while developing a HTML email for a client. If you are testing in Hotmail using a browser other than Internet Explorer, you may notice that the email seems to left align and doesn’t display any background colours or background images the full width of the window. This is because Hotmail’s email container only stretches to fit the width of your most outer table defined with a fixed pixel width.

There is however an easy CSS fix as picked up by Email Center. Simply place the following css in your header:

Center an HTML Email in Hotmail

email, html, center

CSS