The EmailLab boys, Kirill and Loren, have released an email marketing article on Clickz.com about how HTML code can cause your message to be blocked or filtered to the spam/bulk folder. They provide a couple of tested tips to help with this situation.
They also reference a stat from Pivotal Veracity stating that Pivotal Veracity estimates nearly 100 percent of all HTML email doesn’t comply with World Wide Consortium (W3C) standards. I poked around Pivotal Veracity web site without any luck in finding reference to this stat.
W3C has a HTML validator that you can check your HTML emails against to see if they comply with the W3C standards.
eINFO - a collection of articles and studies about email marketing.