Chances are you're attempting to treat an html email as if it were a normal html page and left some of your css in the <head> section that's being stripped out by your email client.
It's hard to say without seeing the code you're working with though.
HTML emails have a different set of rules than normal html pages. Nancy O's post here explains a good portion of it: HTML E-Mail & Newsletter Blasts - http://alt-web.com/
Start with an industry tested HTML email template.
Don't do anything fancy. Most email clients ignore all but the most basic inline styles. A lot of them ignore images, too. So confine your emails to plain text, links and a few images from your remote server. You should be OK.
I think that is exactly the problem. Thanks for the information Jon!
I didn't think about the fact that it could be too fancy for the email clients.Thanks Nancy, I'll be taking a look at those templates!