We have a Outlook 2010/Exchange 2007 environment

If you go to Outlook Properties of a contact and change from "Let Outlook decide the best sending format" to "Send Plain text only", it is supposed to force Outlook to send a Plain text email to that contact.
But it is still sending HTML. This is happening with any contact I try this with.
While composing the email I can change it to Plain text manually and that works fine.
Not sure what gives...
I did try restarting Outlook and I did try starting Outlook in safe mode, where the issue still happened.