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.

Unlimited texting as the norm is pretty new, carriers have charged for incoming and outgoing MMS since it became a thing. Same with minutes and received calls.