I would advise reconsidering keeping the current format exactly. Something that would allow conventional email addresses to kept in a list that disambiguates them unambiguously would probably be a good idea.

You don't want to embed the entire email in JSON. Too many parsers past, present, and future tend to want to decode the entire JSON document into memory before doing anything else, and so you'd be forcing entire email documents to be held in memory at scale, which causes you major problems. I'd recommend something more like either a JSON header that defines what parts are in the email, followed by a normal MIME document, which is perfectly normal in HTTP with other things that use the format as well, or having the top level continue to be a MIME document and specify that the first part must be a JSON document containing the headers. JSON replacing the headers is generally something that would be a good idea, though.

That improves the ability of more language environments to be able to handle the email as a stream or in chunks with "normal" libraries and practices. MIME parsers in languages that tend to favor pulling everything into RAM as a string are still more likely to have been forced to face this issue already.