“The big providers are crushing the field through the sheer amount of resources they can throw at problems, including and especially problems that matter to users like security and spam filtering.”
A university sysadmin who runs his own department’s mail says self-hosting has lost. Spam filtering and security now take more resources than any small shop can bring. So email, the one open, federated system everyone still uses, keeps draining into Google and Microsoft. The protocol is still open. The practice isn’t.