MailChimp is pretty good, I gather, but I haven't used it. I use CampaignMonitor, which lets you import your email template for sending. Not sure if MailChimp allows that, but I would have thought so. With CampaignMonitor, you can even build your template right withing their own template editor, which is very handy if code is not your thing. Other than that, I'd say that maybe MUSE is the problem, not producing code that is portable enough.