There’s at least one piece of data on every call that the network operator has—-the identity of the prior network hop.

In the early days of fighting email spam, that was enough. Every network operator was responsible for receiving spam complaints and responding appropriately. Network providers that couldn’t or wouldn’t prevent spammers from using their networks were blacklisted and other networks would refuse to route mail originating from them.

It wasn’t perfect, but it was far better than nothing.