Simple to fix
Don't accept email from servers that are not guaranteeing authenticity and are willing to accept damage costs in the event of spam relay.
Everyone who knows how email works also knows how easy it is to spoof the sender field, even when they check against the senders IP it can be spoofed. However if all email has to be within the ISP domain and they are responsible for the content an abuser can have their account blocked. All free email services should limit new account post if they want to be able to provide a service.
It would be fairly simple to set up via the existing domain registration system, simply a Y/N question if you accept liability for spam, if you don't then you will not have your email relayed.
Problem solved


