Thanks for the link! I can't see how to do this in Tuta and Tuta doesn't support standard clients because of their encrypted email inbox. Annoying, I have to pick between the email provider not being able to read my emails, or the email being encrypted in transit unless it's between Tuta accounts.
I guess if I use a non-E2E encrypted service and add a GPG key in my clients then I guess this protects it from the provider? But then I have to pick a provider who aren't dicks. Why can't companies just not be dicks.