Bug 858337 implemented header parsing in JSmime. This bug is about handling the message body too in JSMime.
We should add a pref to use it for some simpler type to start off (text/plain, text/html), and then later expand it to other structures.
I'm not sure where a suitable entry point is. Perhaps
See also https://github.com/emailjs/emailjs-mime-parser
See also https://phabricator.services.mozilla.com/D115336