To have any shot at being reasonably portable, your choices are text/plain (and the answer is "no", here), text/html, or whatever the proper mime type is for RTF (application/rtf, maybe?) I'd just as well use html, personally, as its the most obvious of the choices to implement/debug/maintain ... assuming you really need bold. (And since it is 2008, it is a reasonable expectation that a recipient can process html message formatting.)