You have to have a working mail server attached to a domain to be able to send mail... that's the big part. Right now, email can more or less come to anywhere from anywhere as anyone. There are extensions for signing connections, tls, etc... but in general SMTP at it's core is pretty open and there have been efforts to close this.

It would simply close the loop and push the burden of the messages onto the sender's system mostly.

And yes, you can decide from the envelope, and a higher chance of envelope validity.