Can you please give us more details? What version of exchange are you using? What service pack and rollups have you applied please? There are vastly different ways to accomplish things in exchange depending entirely on what version you are using. Thanks!
It’s not easy to set font. First search for .css files are loaded or not, then add a body font style in there (most probably it is premium.css)
goto Tools > options > mail format > fonts or use Word as the email editor in which case u can set the font which sets by normal.dot via word.