There are two free market examples where spam is effectively limited by financial reality.

1. postal mail

2. sms text messages

The basis is in a “sender pays for delivery” model. Note that this is especially highlighted in the case of SMS. In most parts of the world SMS spam does not seem to be a problem because messages are always paid for by the sender. In North America, it can be a problem on those services where the recipient pays, but not those where the sender pays.

Once a sender has to pay for delivery, it completely changes the economic equations for the spammer. Unlimited volumes no longer work because it costs too much to make a profit.