Unfortunately Outlook 2007 composes HTML email with Word 2007 which generates HTML email that may not render as you might expect on anything but an Outlook or Word 2007.
I would bet that if you pull the source for that message out and view it on a standards compliant browser you'll get the same results.
Interesting stuff about Microsoft breaking HTML standards in email here:
Microsoft takes email design back 5 years - Blog - Campaign Monitor Outlook's broken Let's fix it