ProtonMail only keeps the contents of your email private. They can still see the routing information (since, after all, they still have to route it).
You can do just as well by setting Discourse to “private email” mode (found in Admin -> Settings -> Email), which basically strips emails down to a “something happened: go to https://your-forum.example to find out what” notification). Same deal as ProtonMail would give you, where they can’t see the contents of your posts, but it works with every email provider.