Sometimes your email preview can look very different on the website and in your email client; Hotmail, Yahoo, Outlook, Gmail, etc. The root of this difference is that there is no universal standard for what kind of design code an email client should support and how it should go about supporting it. Every email client takes its own approach to rendering HTML and CSS – which is why an email that looks one way in one email client can look completely different in another.
There are ways to optimize your email to render well across all email clients, though. We have a pretty great guide on the Do's and Don'ts of email marketing here: http://help.verticalresponse.com/how-to/tutorial/the_dos_and_donts_of_email_html/
If you're a designer working with custom HTML code, you can reference our HTML Guide at http://www.verticalresponse.com/sites/www.verticalresponse.com/files/html_tips_for_email_guide.pdf
Though we can't support custom HTML code, we can help with tips and advise you on what things can be changed to make your email look better.