Just in time for 10 years of Tuta/Tutanota, we are launching the most significant security upgrade of Tuta Mail with TutaCrypt. This groundbreaking post-quantum encryption protocol will secure emails with a hybrid protocol combining state-of-the-art quantum-safe algorithms with traditional algorithms (AES/ECC) making Tuta Mail the world’s first email provider that can protect emails from quantum computer attacks.
It sounds like they’re just encrypting it twice (once with each algorithm), but I could be wrong.