Bridge is an application for paid users that encrypts and decrypts mail as it enters and leaves your computer, allowing you to integrate your ProtonMail account with your email client, such as Outlook, Apple Mail, or Thunderbird.
Messages exchanged between your ProtonMail account and other ProtonMail accounts while using Bridge are end-to-end encrypted just like they are in our web and mobile apps.
Messages sent to non-ProtonMail 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 ProtonMail account to non-ProtonMail accounts, you must use the outside encryption 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. For those with heightened security concerns, we also recommend encrypting your hard drive. You can find instructions to encrypt your device here: Windows/Mac.