This is a bandaid that avoids solving the harder problem of trust/spam. It is such design patterns that make a fundamentally open/federated protocol more centralized, exacerbating the problem.
Personally, I think the use of proof-of-work like methods can mitigate the problem by a large extent, making it computationally expensive to spam users. This was one of the original goals of what has now become the "blockchain" revolution. Is anyone aware of any projects that are still implementing similar (open) systems?
I am happy to be proven wrong here (not an expert) but IMHO there is not much hope or solving the open decentralised communication problem with email at all. It seems that something like Matrix.org presents much more promise in this area. I also host my own Matrix server, but sadly not everyone I need to communicate with uses Matrix....
Personally, I think the use of proof-of-work like methods can mitigate the problem by a large extent, making it computationally expensive to spam users. This was one of the original goals of what has now become the "blockchain" revolution. Is anyone aware of any projects that are still implementing similar (open) systems?