At least ProtonMail is working towards full compatibility with the OpenPGP standard, thereby allowing non-Protonmail users to encrypt emails using a standard PGP key - according to their stated roadmap, ProtonMail will eventually allow you to export your keypair and will also allow you to upload keys you've generated outside of their service instead of relying on their service to do the key generation.

Tutanota has implemented their own variation of encryption which is not OpenPGP compliant. Their justification for this deviation rests on their desire to be able to encrypt all aspects of communications (metadata, subject, etc) while PGP doesn't not offer this.