Sad but true. The problem is that there are no email standards to speak of. Each email client supports an incomplete set of either HTML and/or CSS making it nearly impossible to determine anything more than a baseline feature set.
Tell me about it. I work for a publishing company that's transitioning from Print to Digital that deals with firms that are even less far along in the transition than we are. So, at least once a month, I have to tell someone, _again_ that we can't use Flash banners in email. We will NEVER be able to use Flash banners in an email. STOP ASKING.