Christel is right as far as getting a masterpage to determine your template. You can see our documentation here on modifying the layout of the message template. Using the available CMS tools is absolutely appropriate for this situation. The same as you would for a page. I have never had a problem styling the email templates so if you want to attach your templates I can take a look at them and figure out what might be causing the problems.
Each email client has its own restrictions for which elements and styles it accepts. The limitations of email campaign HTML is often why people resolve to image-heavy solutions. With that being said Sitefinity's support for these clients is independent. The layout generator works for all of them. Make sure in Outlook your email is trusted or else it won't fully load. Additionally please check this chart to make sure you are using valid markup for each client.
I hope this helps.