ProtonMail bridge has been Open Source since April 2020. It is a daemon that essentialy is an interface to Proton's servers. It can be built with or without it's QT GUI. https://github.com/ProtonMail/proton-bridge/issues https://github.com/ProtonMail/proton-bridge/blob/master/Changelog.md https://github.com/ProtonMail/proton-bridge/wiki ProtonMail/proton-bridge