Ok. I haven't started going into the specifics about the protocol implementation but its about encrypting the message end-to-end. It still does not solve the problem of ownership, privacy and ads.
Having said that, If implemented we would definitely take advantage of the protocol.
Is there any large-scale email service provider who claims they own the contents of your email messages (i.e. own the copyright rights)? All they need is a limited license to do those things they must be able to do in order to provide email service to you (e.g. make a copy on their servers).
As for ads, if the message is encrypted from end-to-end, then even if they read it, they won't find anything useful for ad-targeting. I can already do that today by sending encrypted text via existing email protocols. Moreover, I can use an ad-blocker to block any ads they do try to send me.
The tricky part is keeping things like the address of the sender and the address of the recipient (i.e. the metadata) concealed from eavesdroppers.
Having said that, If implemented we would definitely take advantage of the protocol.