This link went around the office a few days ago about the rendering engine used to display HTML email messages in Outlook 2007. Evidently, the folks from Campaign Monitor are not impressed.
The issue is that Outlook 2007 uses Word to render the HTML email messages instead of Internet Explorer. This leads to some limitations on the HTML and CSS that authors can use in their message.
Zeyad Rajabi and Erika Ehrili wrote this document to help folks understand the rendering capabilities of the Outlook 2007 email client. Its well done and easy to understand.
It looks like a fair number of people are in a tizzy over this. If you make a large percentage of your revenue by designing complex email templates for your marketing clients, you're probably interested in how the email renders in the reader's email client program. Just thinking about the number of email client programs is startling. That's a lot of testing to do before you send out your mass e-mail message. Plus, you have to comply with the federal anti-spam laws. I suppose the marketing folks deserve a little bit of a hassle for all of their networking events and schmooze fests... 
I like a pretty e-mail as much as the next person, provided that its relevant to my interests. I don't like the crippling affects that I see when one is forwarded to me by the original recipient. It seems to me that the message needs to be short, with a clear call to action, and trackable. I'm fine with a details link to a web page.
These are some of the choice excerpts from the comments on the Campaign Monitor page:
jaw - floor.... Where is the frick'n sense in this? Is it 'cuz they didn't employ people who can understand CSS?? WTF??!!
Hey there... I guess this is attrubuted to Vista's. IE has been split from the windows shell thus meaning that if IE is not installed on the computer Internet Explorer cannot render the HTML. See when you are in XP if you type in a url in Explorer it will go to the page "inplace" however if you open up Vista and try to put in the URL windows will open up your default browser to open the page. :)
I like it. I've gotten emails from clients using the background images just b/c they look "pretty" and it ends up destroying the message formatting when I try to reply. Also, it's less junk taking up bandwidth. If you need graphical pretty things sent to specific people, find another way.
Hate to be a spoiler, but I long for the days when my email didn't look like a webpage.
Email should be fast and efficient.
Graphical email is not.
I'm no fan of using the Word rendering engine, but the real problem lies in the overuse of HTML mail. Anyone who doesn't use Outlook already has problems similar to the ones this switch will cause, and there are only two ways to ensure perfect rendering; Use plain text for emails, and use PDFs for pizazz.
I'm almost positive it has to do with the whole Anti-Trust issue. The answer that would make the most sense to me would be the word rendering engine is part of Office. IE is not and therefore instigate the issue that people are being forced to upgrade and use IE versus alternatives. I think its a step in the right direction but i believe that it has hindered the users experiance because of it. Only time will tell if they stick to their guns and give some resolution on it.
I think this is a great move. I'm sick and tired of html email and hopefully this will reduce the number of people that think its cool to put html, background images, and all that other crap in an email.
Currently using Vista and Office 2007 and I do not see these problems problems. I even make HTML based newsletters an I have do do not changes to the way i create them. It looks like FUD to me.
stop crying... BUY A MAC!!!
Hey, maybe now we can get email back to what it's supposed to be. Text-based. HTML email has always been problematic and you've never been really sure about what the other person would see depending on their email client.
Let's talk a little bit about spamminess, and why HTML might not be the way to go in email.
We run SpamAssassin, a requirement when each email account averages 150+ spam messages per day.
HTML is already suspicious and gets -10 -- -40 if HTML only. Since -50 is quarantine, your beautifully crafted HTML only message is precariously close to deletion already.