Please don't violate the guidelines by rudely complaining about downvotes:

https://news.ycombinator.com/newsguidelines.html

Open sourcing the spam filter will only help spammers. It is unlikely that it would work on an individual server with small amount of traffic.

Instead of trying to make a perfect spam filter we should abandon outdated inreliable mail transfer protocol.

What would a spam-resistant successor to today's email protocols look like? Is such a thing in the works by someone?

This is completely off topic from this thread, but one way to easily cut down on spam (that would break many email senders today) would be to require that a mail server can only send email for a domain you own. Yes, I'm familiar with DKIM and similar tools, but they don't go far enough.

Another way is to accept emails only from your contact list.