Designing an HTML email that renders consistently across the major email clients can be very time consuming. Support for even simple CSS varies considerably between clients, and even different versions of the same client. Campaign Monitor has put together a CSS support in email clients guide. With 21 different sets of results, all the major email systems are covered, both desktop applications and webmail.
The Email Standards Project is about working with email client developers and the design community to improve web standards support and accessibility in email.