A user posted this fix today on the Outlook forum. Thanks motoxer913 for sharing!
I finally found the resolution to this insanely annoying problem that exists in Exchange 2010 and Outlook 2011. Basically Exchange is converting all HTML email sent outside the domain to plain text for Outlook 2011 users. My exchange guy thinks this may have been caused during the upgrade from 2003 to 2010. But this powershell script run on Exchange will resolve . . .
Continue Reading: Exchange Mail sent as HTML in Outlook 2011 is received as plain text