Just force the body to be html. I use something like:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta name="generator" content="HTML Tidy for HTML5 for Linux version 5.2.0">
<meta http-equiv='Content-Type' content='text/html; charset=utf-8'>
<title>Title stuff</title>
<style type="text/css">
td.c1 {font-family: Arial, Helvetica, sans-serif; font-weight: bold; font-size: 26px}
</style>
</head>
<body>
this will be HTML
</body>
</html>
And then depending on the email client, the content will be rendered appropriately.
-Garin