The answer is: Bluesky solves this problem via centralization. Since there is really just one very large relay which everyone is expected to participate in, this relay has a god's-eye knowledge base. Entities which sort through mail and relevant replies for users are AppViews, which pull from the relay and also have a god's-eye knowledge base, and also do filtering. So too do any other number of services which participate in the network: they must operate at the level of gods rather than mortals.