Yes, it's a protocol, not a company or platform, the same as NOSTR.
You don't need permission to use it, and nobody can stop you.
Caveats: Individual mail providers can ban you and spam filters may block you and encryption is limited.
Discussion
Plus privacy because the lack of encryption. But then does it worth to fix email, or would it be better to transition to something that is already working?
Like NOSTR?
I guess one way or another yes.
It could keep the "not company" and decentralized, permissionless feature of it.
Or it can be only the keystore for emails. So it would be so much easier to find the keys to encrypt emails to nostr users. Also it could be so much easier to generate these keys.
You can add layers to a protocol definition, but it's nearly impossible to "fix" a protocol.
Email is what it is, you've been able to layer on PGP encryption since its inception in 1991, but most people don't.
So generally it's better to start with a protocol that is built from scratch with the features you want.
I would definitely not change the smtp protocol. I would not even touch it. I would only make an app that makes it easy to encrypt.
But definitely easier if the protocol itself contains the security and privacy features you want.
But there is also the power of already established protocols, that many use.
Maybe the a good way is to somehow have both. Have a bridge, that gives easy access to the new thing via the old, so the transition can be seamless.