Have you tried with other email readers? I do know that text/richtext is now considered obsolete and has been replaced by text/enriched, so you may want to try that.
You may want to try leaving the semicolon off too. It doesn't look like you're adding the Content-Transfer-Encoding header either.
You say 'rtf garbage' - surely you would have to base64 encode the RTF data first as email doesn't support binary data.
As a last point, does Outlook even read RTF data? I thought only text/plain and text/html were supported.