or maybe at least the posts wouldn’t be replicated, but there would only be one community, and when you visit it, posts for the community are downloaded from your primary instance, as well as other instances based on some configuration that makes sense.
SPF broke email forwarding, in order to give us 6 months without spam back in 2014.
Now you need to implement SRS to forward, but it is gennerally easier to pull email from X by Y