> 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

Why? Even if your system loads emails into memory a whole email at a time, that doesn't imply that it will load large numbers of emails simultaneously. The kind of processing that can be done by stream-processing large numbers of emails in parallel seems rather limited to me.

> 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.

If we're specifically talking about being able to short-circuit after headers, then why not a JSON header followed by JSON representation of the body?