From their FAQ -

Can I retrieve my Tutanota emails via IMAP to another email client?

This is not possible as we could not guarantee end-to-end encryption for your data. Instead Tutanota offers email desktop clients for Linux, Mac OS and Windows as well as a web client and apps for Android and iOS.

I don't think that anyone wants them to decrypt the data on their side, but Protonmail has a bridge which translates to standard protocols within the user's domain.