Courts are well practiced at sorting these kinds of things out. They would review the content, presentation, targeting, etc. of the emails to determine if it is commercial speech or political speech.
Color me unsurprised that the end game of the nominally pro–free speech crowd is letting the government determine what legal speech should be censored.
1. Even if the courts always get it right, small sites are not going to want to want to hire a lawyer and go to court to defend someone else's speech. They will instead err on the side of caution (as many do now for DMCA violations), causing a chilling effect on speech.
2. In a system where whether a platform is liable for its content is ultimately decided by courts we cede a lot of power over speech to the government.
I really don't think the courts are the path we want to take.
As courts set precedents, the process will become more streamlined. Ultimately becoming codified into law by congress.