Creating rich text in Mail.app

I am working on an Applescript (Xcode) application that will automate the creation of email-based proofs that we send to our clients. I have a boilerplate file (it is a TextEdit rtf) with simple “codes” (i.e.: “[RECIPIENT]”) that I can do a search-n-replace on. The problem I haven’t been able to overcome is that when I set the content of the message to the contents of the boilerplate file, it is plain text with rtf formatting codes throughout. Is there a way to make the content of the message rich text via applescript?

Thanks in advance,
Brad Bumgarner, CTA