According to SitePoint, Microsoft Breaks HTML Email Rendering in Outlook 2007:
That’s right. Instead of taking advantage of Internet Explorer 7, Outlook 2007 uses the very limited support for HTML and CSS that is built into Word 2007 to display HTML email messages.
Egads! This will be quite a boon for newsletter designers, once they figure out what will work in Word 2007. It will be a horrible pain for the rest of the world. Given Word’s atrocious history of HTML mark-up, I shudder to think what hoops designers will have to employ to get decent rendering.
Perhaps my traditionalist preference for plaintext will come back into vogue.
(Spotted via Simon Willison.)