"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" When t

"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" When thinking about a standard like this, thinking about "your" system is a fatal error. You have to think about all the systems. And, yes, there are systems out there in the world that load large numbers of emails simultaneously. Obviously. Many of them. Not only that, if a new standard can't capture some of those, it's going to be a worthless niche standard forever because those are precisely the nodes that you need to get to support the new standard. "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?" Just because a JSON representation of the body is pretty worthless. MIME is an extremely well-known standard, already used in HTTP contexts, and it will be more efficient than JSON string encoding at a fairly small transition size.