All email clients work differently when it comes to rendering the HTML - what you see in Hotmail, gMail, Outlook 2003 and Outlook 2007 can vary quite a lot... so typically we test emails with each of the email clients before we send emails out - it's the only thing you can do I'm afraid.

What you see in Communicate is  fairly accurate but can't be totally trusted because gMail (for example) will strip out all style sheet styles (apart from inline styles)...

A good guide explaining the differences can be found here - it's quite lenngthy, but does cover all the information you will need.

http://articles.sitepoint.com/article/code-html-email-newsletters

Don't forget: We can build and test your HTML eMail campaigns for you!