HTML Emails can be a nightmare when there are so many different email services, and all seems to operate and render emails quite differently from each other.
Here is an article [1] that should help ease the process
Read the Article - HTML Emails - Taming the Beast [1]
Links:
[1] http://www.thinkvitamin.com/features/design/html-emails
[2] http://duvien.com/node/129
[3] http://duvien.com/how-do-i-search-drupalorg-and-find-what-i-need