Chad Perrin said:
Text email is universal. HTML email alienates a great many people. If I wanted to see HTML, I'd use a browser or an HTML editor.
One very good reason to be wary of html emails is that spammers, and even businesses that think of themselves as reputable, use web bugs, usually image tags to a 1x1 pixel image that cause the email, if read online, to transmit the IP number of the recipient at every time you look at it, alongside identifying information that may permit the spammer (or "reputable" business) to associate IP numbers with email addresses. The commercial value of this information to a spammer is inestimable, and it serves absolutely no legitimate purpose to the recipient. If you must send a html email, please send it multiparted with plaintext so that standards-compliant email systems can choose which version to display.