Proton Mail Bridge

Copyright (c) 2026 Proton AG

This repository holds the Proton Mail Bridge application. For a detailed build information see BUILDS. The license can be found in LICENSE file, for more licensing information see COPYING_NOTES. For contribution policy see CONTRIBUTING.

Description Bridge

Proton Mail Bridge for e-mail clients.

When launched, Bridge will initialize local IMAP/SMTP servers and render its GUI.

To configure an e-mail client, first log in using your Proton Mail credentials. Open your e-mail client and add a new account using the settings which are located in the Bridge GUI. The client will only be able to sync with your Proton Mail account when the Bridge is running, thus the option to start Bridge on startup is enabled by default.

When the main window is closed, Bridge will continue to run in the background.

More details on the public website.

Launcher

The launcher is a binary used to run the Proton Mail Bridge.

The Official distribution of the Proton Mail Bridge application contains both a launcher and the app itself. The launcher is installed in a protected area of the system (i.e. an area accessible only with admin privileges) and is used to run the app. The launcher ensures that nobody tampered with the app's files by verifying their signature using a hardcoded public key. App files are placed in regular userspace and are signed by Proton's private key. This feature enables the app to securely update itself automatically without asking the user for a password.

Keychain

You need to have a keychain in order to run Proton Mail Bridge. On Mac or Windows, Bridge uses native credential managers. On Linux, use secret-service freedesktop.org API (e.g. Gnome keyring) or pass. We are working on allowing other secret services (e.g. KeepassXC), but for now only gnome-keyring is usable without major problems.

Environment Variables

Dev build or run

• APP_VERSION: set the bridge app version used during testing or building
• PROTONMAIL_ENV: when set to dev it is not using Sentry to report crashes
• VERBOSITY: set log level used during test time and by the makefile

Integration testing

• TEST_ENV: set which env to use (fake or live)
• TEST_ACCOUNTS: set JSON file with configured accounts
• TAGS: set build tags for tests
• FEATURES: set feature dir, file or scenario to test

Folders

There are now three types of system folders which Bridge recognises:

	Windows	Mac	Linux	Linux (XDG)
config	%APPDATA%\protonmail\bridge-v3	~/Library/Application Support/protonmail/bridge-v3	~/.config/protonmail/bridge-v3	$XDG_CONFIG_HOME/protonmail/bridge-v3
cache	%LOCALAPPDATA%\protonmail\bridge-v3	~/Library/Caches/protonmail/bridge-v3	~/.cache/protonmail/bridge-v3	$XDG_CACHE_HOME/protonmail/bridge-v3
data	%APPDATA%\protonmail\bridge-v3	~/Library/Application Support/protonmail/bridge-v3	~/.local/share/protonmail/bridge-v3	$XDG_DATA_HOME/protonmail/bridge-v3
temp	%LOCALAPPDATA%\Temp	$TMPDIR if non-empty, else /tmp	$TMPDIR if non-empty, else /tmp	$TMPDIR if non-empty, else /tmp

Files

	Base Dir	Path
bridge lock file	cache	bridge.lock
bridge-gui lock file	cache	bridge-gui.lock
vault	config	vault.enc
gRPC server json	config	grpcServerConfig.json
gRPC client json	config	grpcClientConfig_.json
gRPC Focus server json	config	grpcFocusServerConfig.json
Logs	data	logs
gluon DB	data	gluon/backend/db
gluon messages	data	gluon/backend/store
Update files	data	updates
sentry cache	data	sentry_cache
Mac/Linux File Socket	temp	bridge{4_DIGITS}