most email clients cannot use Embedded or External CSS styles. in emails, you need to inline styles rather than class names.
remember that email clients are not web browsers, so the same rules don't apply.
edit the Universal Email behavior that is on the page, and edit the body by clicking the DW Edit icon.
remove the existing display manager from the email bodt and replace it with a new one, in the display manager type, select to use the email Friendly version, this will add the style inline instead of using external css with classes.