I've never been a fan of the Calibri default font for Microsoft's 2010 office products. Instead, I prefer using a serif font, typically opting for Century Schoolbook. Changing the Styles in Microsoft Word is the most long-term efficient way of setting the default font. But, what about Outlook. The answer is not as obvious as it is in Word.
To set a different default font in Outlook, go to the File tab and choose options. On the left side of this window, choose Mail. Then, scroll down and push find the Stationery and Fonts button. From there, you can choose from various themes or set the default fonts you want.