Outlook 2007 will 'remember' what format it sent to an address. You need to make sure that it doesn't send in "Outlook Rich Text Format".
How to set that email address to not send:
1.) Make sure your Outlook Options are set correctly: click Tools: Options: Mail Format: Internet Format... and make sure that it is set to "Convert to HTML format"
2.) Open a new message in Outlook and type in the email address, then right mouse click on that address and select "Outlook Properties":
3.) Make sure that you set the "Internet Format" to "Let Outlook Decide the best sending Format":