@resist1984 @Tutanota I am not an expert and I may be wrong but that's probably due to Tutanota not using PGP. They encrypt emails using a custom method for reasons they explained here: https://tutanota.com/faq#pgp. On the other hand, ProtonMail uses PGP. This is one of the key differences between these two great services.
@resist1984 Thanks for your feedback. Yes, this is currently not possible as we don't use PGP, but we plan to support Autocrypt in the near future.
@Tutanota until then, I'll write my #Tutanota gripe right here: the command "gpg --auto-key-locate clear,nodefault,wkd --locate-key $tutanota_address" gets no results. It works for #Protonmail users. Plz make it happen.