Why won't Hotmail show HTML emails correctly?

I'm getting very odd behavior from Hotmail addresses when sending HTML emails via Simplenews and Mimemail, namely, the message comes through as a plain-text version, with the original included as an attachment. Messages sent to gmail addresses are normal, with links working as they should. What's going on? Is Hotmail simply too picky and won't let a html-themed message through? Since the unsubscription link is featured in the footer of every Simplenews message, I hope some solution could be found.