The only methods of enforcement between sites were either personal favors or the nuclear option of not peering.
But, The Cabal (which does not exist,) would smack people down if they became too outrageous. Not mere assholes, but people who ruin the commons for everyone...
I'm talking about Spammers.
Back in the day, email was expected to allow for forwarding, and there was no difference between the protocol for me sending email to my server, and my server sending email to your server.