The difference is that email is private. The contents of your email are not seen as being endorsed by Google just because you have an @gmail.com address, so Google doesn't have an incentive to ban you just because they find your politics odious.

In the fediverse calls to defederate happen pretty regularly, and the reasoning is sound—your instance will be publicly mirroring any content on a federated instance, so if you don't want to publicly host extremist views (from either wing) your only option is to defederate from instances that allow them.

Banning in the email world means having your messages dumped into a junk folder. Or worse having your IP address in a reject list so its impossible to operate a private server.

And those would be good arguments, if the SMTP landscape looked different. I think the OP's views would be best summarized by "either host your own server, or go back to Reddit". Similarly, you shouldn't be using random-joe.biz as your SMTP host. You should use Google, Fastmail, or a handful of other reputable providers you can probably count on one hand. Or host your own server.

Reputable does not mean large, and (more importantly), large does not mean reputable.