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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?

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    Have you read their FAQ on this?
    – Euphrasius von der Hummelwiese
    Mar 7, 2019 at 6:29
  • @EuphrasiusvonderHummelwiese This ONLY applies to providers that support a basic encryption standard but not the level ProtonMAil does, Gmail, Yahoo, and the major others support the more basic TLS. But I'm asking about ProtonMail to TutaNota which both try there hardest to provide end to end encryption.
    – Ultra Gamer
    Mar 7, 2019 at 18:12
  • I believe the FAQs are accurate and I cannot find a statement making different claims, neither on Protonmail, nor on Tutanota: Between ProtonMail users there is E2EE and nowhere else (except for the Encrypt for Outside-Option).
    – Euphrasius von der Hummelwiese
    Mar 8, 2019 at 6:31
  • @EuphrasiusvonderHummelwiese I guess that makes sense considering neither service would have access to each other's encryption keys, thank you
    – Ultra Gamer
    Mar 9, 2019 at 6:26
  • @EuphrasiusvonderHummelwiese Would you mind writing a full answer on this just explaining that so that I can mark this as answered? It would give you points and it would take this off the "unanswered" list. I don't need you to give any more detail than you already have.
    – Ultra Gamer
    Mar 29, 2019 at 3:10

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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.
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