My doctor uses Paubox to encrypt emails but I have no idea how it works

Years ago I did a bit of mail administration so I understand enough about SMTP and TLS to be dangerous. That being said, it's unclear how my doctor, who uses Gsuite, integrates Paubox to encrypt emails from end to end. Does Paubox just ensure TLS encryption is enforced by both parties by acting as a man in the middle SMTP relay? Is that all there is to it?