That’s probably an eventual step. I think the left will go after the ISPs first since those have more of a natural monopoly on services in their respective markets. Kind of like how Google and Facebook hold a de facto Duopoly over the online advertising space. There aren’t nearly as many ISPs as there are email service providers and the parent companies of the ones like Comcast and AT&T are already pretty publicly on the side of censorship. They’re a much easier target.