Ouch.. been there! Me and a friend thought up a nice e-stationary tool, untill we hit the walls of microsoft. It even got worse,
they took the HTML posibillities for email back 5 years. It's important to me since i also do newsletters for my clients. I know that some are pro and some con towards HTML-mailings, still.. considering xHTML and CSS it shouldn't be this bad!
If you can, consider
Thunderbird. I love using it. I know some clients still use the microsoft junk for reasons i can't really comprehend.