Well one issue with them is that they're totally optional. The protocol should have this built in. The way it is now is a kludge. Even mails from big companies

Well one issue with them is that they're totally optional. The protocol should have this built in. The way it is now is a kludge. Even mails from big companies still end up in my O365 junk box. I didn't check them in detail but something failed there. At the same time the spammers are actually pretty good at avoiding spam filters. Also, they don't really provide sender authentication. They provide sending domain->sending server verification. If you have a business that uses the same email provider, you will be able to fake sender auth if you can convince the server to send it. This is why it's a kludge. If we'd require emails to be signed by default this issue wouldn't be present. What I really miss these days is public protocol innovation. In the old days if a protocol stopped meeting the needs, people would go back to the drawing board and publish and agree a new RFC. These days we just tack on workarounds to old protocols without fixing the root issues, or we switch to closed alternatives (like what has happened to IM) which is even worse.