Proton forces you to pay for a bridge to use Thunderbird.
Tutanota doesn’t even provide that.
These “privacy respecting” email services don’t respect the user enough to let them use third party email clients easily if the user chooses to.
Go ahead and explain what you mean. I don’t believe you & think you’re just parroting their corpo speak.
It’s actually fairly simple: if the server never has access to the keys or the plaintext of messages (or calendar events, etc.), then you need a client tool to handle decryption and encryption operations.
They use PGP, and they have implemented this feature in a way that it’s completely transparent to the user to make it mainstream. So they chose building dedicated tools (bridge, web client), rather than letting users use their own tools, because the PGP tooling sucks hard and it’s extremely inaccessible for the general population.
This means that you need a fat client, whatever you do, or otherwise the server will have access to the data and there is no e2ee. Instead of using enigmail or other PGP plugins/tools, they built the bridge.