The solution is allow only communications that are demonstrably not spam and not unwanted.
There have been lots of proposals to this effect but maybe you have to put up two cents in a crypto wallet, which the other person can claim if the mail is unwanted. If someone takes your two cents, you remove them from your lists ASAP. The receiver doesn't benefit meaningfully from the 2c so they are unlikely to claim it. The sender is only damaged if a lot of different people claim the 2 cents. Maybe a few domains are default whitelisted even if they don't put up 2 cents, to protect against entities that are important but would be frequently spite-claimed (IRS, courts, and other similar). If you use a provider that doesn't whitelist, fine, but you're going to miss those emails.
Maybe the top 'spite claim' users are published and it becomes difficult to get people to agree to put up money to talk to you.
This reduces spam to the level of junk mail, which is still not totally trivial but is orders of magnitude more manageable.
There is and it's the same as other protocols including proprietary ones - a sender address whitelist.
There's no proprietary protocol that solved spam either.
Seems they are the disease not the cure
It seems insane that gmail doesnt provide a way to automatically send a mail (eg based on domain, keywords, etc) to spam. I know theres block sender but for example aliexpress uses a different name for every email so that's useless.
What this is called is survivorship bias. The question I would ask is how much mail are you not seeing at all because it's dropped at the edge first thing because it's IP block listed?
Next, how much spam are you seeing from them because they and Google are the largest providers on Earth? Kind of like selecting a random person from the population in the US and going "look, it's a white guy". Yea, that's how statistics works.
Lastly, because the are good at blocking spam at the edge, then that's the last bastion of attack for spammers to put their efforts in if they want any return at all.
These entities communicate via plain old mail, that costs more than 2 cents already.