Runbox is more like a "normal, ordinary" e-mail service with high security & privacy precautions, but for example no integrated end-to-end, zero-knowledge encry

Runbox is more like a "normal, ordinary" e-mail service with high security & privacy precautions, but for example no integrated end-to-end, zero-knowledge encryption (option) for mails. You can of course use pgp/gpg like with any other service. So in Protonmail, for mails inside the service, from protonmail user to protonmail user, they afaik use a pgp implementation and handle the key exchange. To send secure messages from protonmail to another service, you can encrypt the message / conversation with a password and the recipient gets an email with an url, where he can, given he has the password, decrypt the message and reply securely.