Muttdown is a sendmail-replacement designed for use with the mutt email client which will transparently compile annotated text/plain mail into text/html using the Markdown standard. It will recursively walk the MIME tree and compile any text/plain or text/markdown part which begins with the sigil "!m" into Markdown, which it will insert alongside the original in a multipart/alternative container.

It's also smart enough not to break multipart/signed