Yeah, even requiring users to _send_ messages probably isn't enough to prevent spam. But that's assuming something like the example I imagined, someone wanting

Yeah, even requiring users to _send_ messages probably isn't enough to prevent spam. But that's assuming something like the example I imagined, someone wanting to received encrypted messages from anonymous others. If a user's address is private tho, then asking them to pay to receive messages should be as sensible as the degree to which they protect the distribution of their address.