Email clients are very limited in their HTML capabilities, and even more limited when it comes to CSS. However, the web browsers you use to preview your emails, or view them as a web page, are not.
How are you creating your emails - are you using an editor like Dreamweaver? Such editors usually design for web pages - not emails.
The way you need to design your email is with basic nested tables. Try to use most basic HTML to format your email as much as you can. Then use only in-line CSS to refine it to perfection. This technique ensures that the look of your email will deterioate gracefully in lesser capable email clients.
Do not use <div> containers, or colspan and rowspan in table cells. No css between the <head> tags, and no css floats or css positioning. There are many articles on the web about designing HTML for emails.
You may also use tools such as emailonacid.com to preview how your email will look in different email clients. This can be a real eye-opener!
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