I think Outlook is correct. The top-level content type (in the headers) is multipart.

If you want a plain HTML mail (and you should really send multipart/mixed(text/plain + text/html)) then you should create it that way to start with. The headers suggest that you're creating the mail programatically. If you don't like the way it has been created then my advice would be to take a look at that and do it differently...