Email Clients will evaluate CSS differently and thus can change the way an email is displayed when received.  Be sure to to include email addresses with all of the major email clients (e.g. Yahoo, Gmail, etc.) to ensure that the layout is as you want it as best as possible. Additionally, below is a source for tracking how different email clients will evaluate CSS can be found on the following third party resource:

Campaign Monitor
http://www.campaignmonitor.com/css/

There you can use this resource to see how email clients evaluate the CSS and how they handle each type in the handy chart that is offered via the above link.