The ProtonMail Bridge is an application for paid users that runs on your computer in the background and seamlessly encrypts and decrypts your mail as it enters and leaves your computer. It allows for full integration of your ProtonMail account with any program that supports IMAP and SMTP such as Microsoft Outlook, Mozilla Thunderbird and Apple Mail. Once you've installed and adder your account(s) to the Bridge you will need to configure your email client. The setup guide for Thunderbird can be found here: https://protonmail.com/bridge/thunderbird