Isn't there any open source project that solves the e-mail delivery problem? If not, why not? This sounds like something that can be fixed by software.
Aplogies if you do, but it sounds like you misunderstand how email works.
The problems are stricly in policy, not in software. So there is nothing an open source project can do. The problem is that "too big to fail" megacorporations like microsoft just randomly decide to block incoming email from most of the Internet.
If email was fully decentralized (everyone runs their own server) this centralized power could not exist and there wouldn't be any problem.
(That all said, I run my own email infrastructure since long ago and it works fine. But I know some people struggle, which is contrary to the intent of the Internet.)
In today's world when microsoft or gmail or yahoo start misbehaving in email handling, what are you going to do? It is impossible to reach them and while you could block them, they will never notice and you're only hurting yourself.
It is the classic problem of centralized power which hurts everyone except the (near-)monopolists.
You would have different problems though. Spam would still be hard, or arguably even harder, because you would have lots of small mail servers with no idea if they are trustworthy or not. Is this uncle Bob, who I havent spoken to for 30 years, sending me a heartfelt message about the family? Or is it a scammer trying to cream some cash out of me?
Doesn't make any difference. The major sources of spam today are places like gmail and outlook servers. You have to run some message classification regardless of where it came from.
By volume perhaps, but is that true as a proportion of all mail that was sent via these services? I'd be surprised, though I'd be very interested if there where any numbers publicly available.
Would you care? I can say that I don't care (as someone running my own email infrastructure).
I run filtering on all incoming email, I don't assign any weight to which email server it came from because it's not a useful factor.