I am sure that Tutanota does not use any custom encryption algorithm. It is clearly stated in the FAQ that they use RSA (with PFS) and AES to encrypt emails exchanged between Tutanota users.
https://tutanota.com/encryption
There’s even a section which discusses why they do not use PGP. So it’s not like they can’t add it, they just don’t because it lacks “important requirements”. Plus they even are slowly developing a protocol that is post-quantum secure to encrypt their emails with.
I am sure that Tutanota does not use any custom encryption algorithm. It is clearly stated in the FAQ that they use RSA (with PFS) and AES to encrypt emails exchanged between Tutanota users. https://tutanota.com/encryption There’s even a section which discusses why they do not use PGP. So it’s not like they can’t add it, they just don’t because it lacks “important requirements”. Plus they even are slowly developing a protocol that is post-quantum secure to encrypt their emails with.