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.
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.