So I know that sending emails from a ProtonMail Email to a ProtonMail email is end-to-end encrypted. And I know that sending emails from a Tutanota Email to a Tutanota email is end-to-end encrypted. I also know that both services can send to other providers such as Gmail but then Google can read the message through the person it was sent to. But if I have a ProtonMail email, and my recipient has a Tutanota email, how much data is exposed compared to sending an email from a ProtonMail email to a Gmail?
No. They are not.
Citing the official FAQ from ProtonMail:
Emails sent between ProtonMail users
- Always end-to-end encrypted.
Emails from ProtonMail users to non-ProtonMail users
- End-to-end encrypted if the “Encrypt for Outside” option is selected.
- Otherwise encrypted with TLS if the non-ProtonMail mail server supports it (most providers such as Gmail, Yahoo, Hotmail, etc, support TLS).
- Note, since these messages are encrypted, but not end-to-end encrypted, this means Gmail, Yahoo, Hotmail, etc, will be able to read these messages and hand them over. This is not possible if “Encrypt for Outside” is set and ProtonMail end-to-end encryption is enabled.