Online email clients such as Gmail, Yahoo! Mail, and Windows Live may render differently depending on the web browser and operating system they are being used with. Designing an email and only testing it for Gmail within Safari, or Outlook 2007, is not adequate. It must be verified within all possible viewing contexts. For example, Outlook is fairly incompatible with < div > tags when used to align specific elements left or right inside of a containing centered < div >. Outlook does not view the < div > tags in the same way as Gmail, Yahoo! Mail, or even Outlook 2003.
The solution to this problem is to use building tables when coding HTML for emails, creating code as a lowest common denominator to viewable media, and to use very little (if any) CSS.