They'll almost certainly still be able to block open mail relays, but it sounds like port 80 is about to be set free. Expect carrier-grade NAT to become even more common in retaliation.
IPv6 exists and works, and if we're going to expect our ISPs to get with the times we should be prepared to do so ourselves. NAT for IPv4 is ultimately unavoidable and thus excusable, but that doesn't mean that they can't use it to make the transition to IPv6 rougher on people who still need IPv4.
On what grounds? Open mail relays certainly get abused, but there's nothing inherently illegal about them or the traffic they serve.