When you request a preview of your email message in Flexmail then you are viewing your message in your standard browser (Explorer, Firefox, Chrome, or other).
Your email programme will process the html code of your email differently than your browser. What’s more, every email program will process the html code in your message in a different way. An email message will look different in Outlook, compared with Apple Mail or Gmail.
Solve this by using the limited html code and style sheets supported by all the email clients.
You can find some CSS tips here.