I'm running Outlook 2011 14.3.5 (130515).

When I send HTML Messages, the recipient will get the message in Plan-Text. Yes the HTML switch is enabled. I can paste in images, formatted text, etc. 

I ran though several tests with sending to a vendor and my Yahoo Account. The Mac Client is sending as plain text after it leaves the exchange server The PC Client is sending as HTML. 

Inter office email is received as HTML. 

Thank you,