The other day, Smashing Magazine tweeted about an updated guide to CSS implementation in e-mail clients. The guide is extremely helpful, as it shows you exactly what CSS techniques and features you can use within your HTML e-mail messages and which e-mail clients will display them correctly.
The chart displayed in the original blog post shows over 50 different CSS techniques and elements are evaluated for 11 different e-mail/Webmail clients. Apparently the PDF and Excel versions of the chart that are linked from the post include more than 23 different mail clients. Unfortunately, looking at the chart, it appears that the safest way to create an attractive e-mail message that looks the way you intend in multiple e-mail clients is to code with tables and inline styles.