Proton Bridge is Proton's official GPL-3.0 desktop app that exposes a local IMAP/SMTP server so standard email clients can use end-to-end-encrypted Proton Mail.
Yes, proton-bridge is open source under the GPL-3.0 license.
proton-bridge is open source: self-hosting is free, and you pay only for your own infrastructure.
Yes, proton-bridge can be self-hosted (the source is available under the GPL-3.0 license).
Proton Bridge is Proton's official desktop application that runs a local IMAP/SMTP server on your machine, letting standard mail clients — Outlook, Thunderbird, Apple Mail — send and receive Proton Mail. It decrypts and re-encrypts messages locally after you authenticate with your Proton login, so end-to-end encryption is preserved while the client sees ordinary IMAP/SMTP. Written in Go and GPL-3.0 licensed.
Because it is the sanctioned, first-party integration path (unlike reverse-engineered alternatives such as hydroxide), there is no terms-of-service risk. Proton's own product documentation ties Bridge to paid Proton Mail plans; that requirement is not restated in the GitHub README, so confirm current plan eligibility with Proton before relying on it.