We have a system (PHP) that sends HTML formatted mail to an email address that resides on our Exchange server. The Exchange server delivers that mail to a public folder. When the mail is delivered there, it appears that the HTML formatting is stripped (tables, specifically, are removed).
When the same message is delivered to a "normal" mailbox the formatting is fine.
I've confirmed this in Outlook 2007, Entourage, and Outlook Web Access.