Anyone familiar with XHTML needs to go back and start using HTML (old schooled). It is more like going back to the older version of HTML where designers and developers used a lot of TABLE. Whereas, XHTML uses positioning unlike : float, which might be a wrong idea for email template designs.
Although, the email template uses a lot of TABLE there is a risk of nesting which can create an issue with different browsers. Never mind !! be prepared to do lots and lots of cross browser testing. Anything that appears good might turn out be a big disaster in other browsers.
What a pain huh?? Don't worry it is not just you, even the web developers who has a practice of many years in web development can go wrong with the web template design. Be prepared to make a lot of mistake and learn. Grabbing a cup of tea or coffee might be a good idea.
Therefore, simplicity is the key issue. Keep the design to minimal and simple that reflects the content. Remember!! content is the king.
For more help go and visit: http://net.tutsplus.com/tutorials/html-css-techniques/20-email-design-best-practices-and-resources-for-beginners/