1. I register a domain using a service in a country outside of the EU (since Posteo is German).
2. I also apply a "privacy protection" service, which means my data won't be visible in the public whois (the domain will be officially registered by the privacy service that I use).
3. I sign up to Posteo and configure my third-party domain.

This means Posteo doesn't know anything about me, and even if the police raids Posteo they can't find who I am.

Of course they could, in theory, find out who I am, but that means going to the provider that I used to sign up for my domain, which may be on the other side of the planet outside of German police jurisdiction.