The level of css support in various different email clients varies massively over the 20+ major clients.

Our fiends over at Campaign Monitor have created this guide to the level of css support that each client has.

Interesting, but rather not surprsingly, Outlook 2007 is a big step backwards in its level of css support over Outlook 2003.

Take a look, it makes for a rather depressing read

Guide to css support in email clients 2009