how does this compare to protonmail ?
So if you use protonmail and your colleague uses this, and you want to have a secure conversation over email, you're each going to be clicking clicking a fuck load of "view this message on <service you don't use>" links.
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.