We've had trouble using RTF in our company. I'm not sure, but I think that starting around IE v5.5, Outlook Express quit recognizing the RTF format properly. Outlook users would send attachments in emails formatted with RTF and the attachment wouldn't come across to the Outlook Express user. This was never a problem before. Microsoft says that Outlook Express simply doesn't handle RTF. Gee thanks.
I'd avoid RTF format.