Depends on your thread model: Swiss police can see your proton mail if they get a court to allow viewing it. But the Swiss do not have a submarine, so underwat

Depends on your thread model: Swiss police can see your proton mail if they get a court to allow viewing it. But the Swiss do not have a submarine, so underwater bottle passing is safe against them. Combine both, and you are safe! Offline mails in a bottle should be a april fool's RFC any time now.