Everything you've ever wanted to know about HTML email
For people that have never had the "joy" of making fancy HTML emails, it's a dangerous world out there. HTML emails are messy and get complicated. Tables are required quite often, and it's a LOT more painful than IE6 ever was. Not only do you have to deal with different rendering engines, ever email client also treats these emails differently. Outlook 2010 does things a bit differently to 2007, which is different to Thunderbird. Web-based clients are even worse. Not only do you have to deal with the cross-browser issues, you also have to deal with the email provider. Gmail, Yahoo and Hotmail all mangle HTML emails in different ways. Hotmail especially - You often need to include some "bug fix" CSS to work around Hotmail issues.
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tl;dr: Use plain text emails.