Comment by dlcarrier
1 day ago
Email has federated pretty reliably. Granted, it's nearly impossible to host your own server, because the rest will automatically assume you are only sending spam, but the well-known servers all federate quite nicely. Worst case scenario is that some organizations put a footer on outside messages warning that it's not internal.
Federation is much easier when you only need to see messages explicitly sent to you, rather than aggregate content from all over the world.
Usenet handles it fine, and it predates federated email.