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 the problem. The problem was the contenttype on the exchange server was set to MimeText and should be set to mimehtmltext.
1 "get-remotedomain | set-remotedomain -ContentType MimeHtmlText"
Source of the fix risualblogs HTML Mails sent Via OWA and Outlook 2011 are received as Plain Text mails externally
One of the comments indicated this worked for Exchange 2007 SP3.