jmtd → log → mml
This morning I discovered Mail Markup Language which is exactly that. Unfortunately, it appears to have some outright bizarre features. The RFC draft describing it is enormous, including a full XSLT description of the markup language (and not in an appendix either); a legal preface claiming that the entire thing was patent pending (and using a subset of MML would be considered patent infringement); it claimes to obsolete 1939, 3501, 5322, which are POP3, IMAP4r1, and Internet Message Format respectively.
You shouldn't judge a book by it's cover, but the fact their website displays awfully for me (epiphany 2.22.x) did prepare me somewhat for the zaniness that followed.