Personally, I have long maintained that HTML belongs in browsers, not my email client. One of the reasons I use Mailsmith is that it never shows HTML in my email, stripping it in to plain text, if possible, and at worst keeping it as an HTML attachment I can open in my browser.
Scot Hacker wrote an excellent article that sums up all of my reasons why you shouldn’t use HTML in your email, and he offers tips on several email clients/services for turning HTML formatting off. Bookmark this one, boys and girls. (Thanks, Lee!)