I know I'm REALLY late to the game on this one, but I just today found out about Microsoft taking a huge step backward with their HTML rendering engine for Outlook 2007 and felt the InterTubes deserved to have my little bit of rage added to the universal panning that seems to be the world's opinion of it.
I just wrote a little application that sends an html formatted email. The app will only send to one person, so it didn't need to be too extensively tested for email client compatability. I tested it with Windows Live Mail, Hotmail, and Thunderbird. All was well.
I then sent the link to someone else to have a look at, and was told it basically looked like garbage. She re-checked it using Yahoo's online email. Everything was peachy.
I had her forward me the email from Outlook 2007 and looked at the source and found this:
<meta name=Generator content="Microsoft Word 12 (filtered medium)">
Now where did that come from? Well I went Googling, and that's when I found out that Microsoft replaced their Internet Explorer based rendering engine with Microsoft Word 2007 for Outlook 2007 and essentially crippled the ability to properly display html formatted emails.
I checked the original email in Outlook 2003, and while not all the formatting was in place, it was a heck of a lot better than what Outlook 2007 produced.
So thanks a lot, Microsoft, for taking a big backwards step and making life difficult for web developers all over the world.
Here's a Microsoft page telling developers how they can try to accomodate their crappy product. I particularly enjoyed the long list of comments at the end of it.