Receiving email, the service provider has full access to the metadata agreed. The main difference between proton and tuta is what data is kept encrypted at rest.

Proton does not encrypt the metadata, from, too, subject

Tuta does encrypt all of that metadata at rest

The clients are open source, you can do anything you want, you just have to implement it. I don’t know where the hate is coming from. Tuta is unique being the only email provider that encrypts all the data at rest, and I want to give them a lot of love for that, I don’t understand the hate at all