If every email service prohibits a topic, that's indistinguishable from state censorship. I fail to see how only one is bad and the other isn't.
The free market isn't of any help here either. Censorship is about stopping the spread of information and ideas. What's not known can't affect the free market.
It's distinguishable because nothing stops you from starting a new email service if you want to. You're not going to end up in jail if you start an emailing service for Blockchain projects. In many countries journalists and authors do end up in jail (or worse) because of actual censorship.
If nobody wants to host your content then maybe the problem isn't them, maybe it's you. And if it's not then there's a business opportunity waiting for you right here.
The free market isn't of any help here either. Censorship is about stopping the spread of information and ideas. What's not known can't affect the free market.