HTML is for the web, not for e-mail. If you find someone sending e-mail messages with HTML in them, chastise them up and down and tell them how to disable it. Nevertheless, there are plenty of jokers out there sending web pages around masquerading as e-mail messages, and you need a way to read them. Fortunately, Mail-It makes this easy.
If you get a message including HTML, just pull down File | Show As HTML, or click the Show as HTML button at the top right of the message (if you haven’t chosen to hide the Mail-It toolbar in the preferences) and the message will be launched directly into the default web browser, which is usually NetPositive.