I thought I should note I experienced the same problem. The e-mail comes to Outlook 2003 as text. Additionally, the FROM line on the e-mail is empty.
On the other hand, it went through as HTML to a yahoo account.
I subsequently made the changes to line 114 of the mail_PHP.php file, as Ray suggested, and the e-mail I receive in Outlook 2003 is okay now. (Thank you, Ray, for the fix. I thought I was going mad.)
I am mentioning this primarily for the benefit those in charge of scheduling updates