How HTML Validity Affects E-Mail Deliverability

ClickZ has a good article on how HTML Validity affects e-mail deliverability.

The article points out that AOL is checking for valid HTML in email messages, and will reject mail with a 554 HVU:FO error response.

AOL HVU:FO Error:
All or part of the HTML contained in your message is improperly formatted. Please make sure that all HTML in your message is properly formatted to industry standards and try your message again.

Some other things the article points out:

• A tracking beacon below the closing HTML tag will get e-mail filtered to the bulk folder at MSN/Hotmail.
• Using %20 in URL's will cause your e-mail to be blocked or sent to the bulk folder at AOL, CompuServe, and MSN/Hotmail.
• Using a decoy link that shows one URL in the e-mail but actually redirects to another URL when clicked also gets e-mail directed to the bulk folder on MSN/Hotmail.
• A poorly constructed boundary between the text and HTML portions of a multipart e-mail message also sends the e-mail to the bulk folder at MSN/Hotmail.

Some other things to keep in mind, is that CSS support in email clients is not the greatest. See this article on A List Apart, for more info on this.

Also keep in mind that the HTML to content ratio can also cause your email to be marked as spam. Spam assassin for instance, adds to your spam score based on this ratio.

You can see from a marketing perspective how important it is to write valid HTML code. Not only will search engines penalize you for it, but email filters will as well.