Proton Mail Bridge is a desktop application for paid users that runs in
the background, encrypting and decrypting messages as they enter and
leave 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://proton.me/support/protonmail-bridge-clients-windows-thunderbird

This script repackages the Debian binary provided by Proton AG.
