Proton Mail Bridge(new window) is an application for paid users that encrypts and decrypts mail as it enters and leaves your computer, allowing you to integrate your Proton Mail(new window) account with your email client, such as Outlook, Apple Mail, or Thunderbird.
Messages exchanged between your Proton Mail account and other Proton Mail accounts while using Bridge are end-to-end encrypted(new window) just like they are in our web and mobile apps.
Messages sent to non-Proton Mail accounts are not end-to-end encrypted, though your emails are protected by zero-access encryption while on our servers. To use end-to-end encryption when sending from your Proton Mail account to non-Proton Mail accounts, you must use our Password-protected Emails(new window) feature in our web and mobile apps.
Keep in mind that your messages are stored locally on your computer in plain text. For this reason, it is important to protect your device(new window).