Don't forget that Outlook's message window is essentially Internet Explorer,
Actually Outlook's message windows are actually Word, Word being a power unto itself. That's why web formatted or html pages don't load correctly in word, because it never received the attention IE did. It's also why some exploits remained in Outlook/Express despite IE seeing patches early. I think it's more or less caught up in the security department now, especially since XPSP2, but Outlook 2007's display output is horribly broken for html messages, because they yoinked out the IE hooks at the last moment and replaced the viewer with the old Word one. <sighs>