Experience shows that the walls will get taller and the moats will get deeper.
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
These entities communicate via plain old mail, that costs more than 2 cents already.
We have dozens of different ways to send E2EE messages to each other, with realtime publish-subscribe, and yet billions of dollars are spent trying to duct tape email. Who cares if Outlook/Gmail/Icloud use IMAP or Websocket? If the interface looks the same to the end user will they really care?
Experience shows that the walls will get taller and the moats will get deeper.
The people working on outlook.com are just talentless amateurs - the rest knows how to do it right. It's a shame it's so widely used.
What about tencent.com, foxmail.com, or qq.com?
Most of them wouldn't even care probably...
Fortunate for those running their own servers is that they actually still get all the cool emails from people not complying.
At the most you need to use an external service for delivery (usually because you have problems with MS) but you can still run your own server with no difficulties once you do that.
That said, DMARC is required by a lot of people now, so I do not think this particular issue is a problem.
I have been self hosting my my personal email (and for family). Few problems so far other than delivery to MS and I have recently moved a domain I use for work email over.
Oddly enough the MS problems come from hotmail, but not other services. If that is a problem, then pay a bit to have email delivered.