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Proton Mail Bridge and Import Export app

Copyright (c) 2022 Proton AG

This repository holds the Proton Mail Bridge and the Proton Mail Import-Export applications. 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, firstly 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.

Description Import-Export app

Proton Mail Import-Export app for importing and exporting messages.

To transfer messages, firstly log in using your Proton Mail credentials. For import, expand your account, and pick the address to which to import messages from IMAP server or local EML or MBOX files. For export, pick the whole account or only a specific address. Then, in both cases, configure transfer rules (match source and target mailboxes, set time range limits and so on) and hit start. Once the transfer is complete, check the results.

More details on the public website.

The Import-Export app is developed in separate branch master-ie.


Launchers are binaries used to run the Proton Mail Bridge or Import-Export apps.

Official distributions of the Proton Mail Bridge and Import-Export apps contain 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.


You need to have a keychain in order to run the Proton Mail Bridge. On Mac or Windows, Bridge uses native credential managers. On Linux, use secret-service 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

Bridge application

  • BRIDGESTRICTMODE: tells bridge to turn on bbolt's "strict mode" which checks the database after every Commit. Set to 1 to enable.

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



The database stores metadata necessary for presenting messages and mailboxes to an email client:

  • Linux: ~/.cache/protonmail/bridge/<cacheVersion>/mailbox-<userID>.db (unless XDG_CACHE_HOME is set, in which case that is used as your ~)
  • macOS: ~/Library/Caches/protonmail/bridge/<cacheVersion>/mailbox-<userID>.db
  • Windows: %LOCALAPPDATA%\protonmail\bridge\<cacheVersion>\mailbox-<userID>.db


User preferences are stored in json at the following location:

  • Linux: ~/.config/protonmail/bridge/prefs.json
  • macOS: ~/Library/ApplicationSupport/protonmail/bridge/prefs.json
  • Windows: %APPDATA%\protonmail\bridge\prefs.json

IMAP Cache

The currently subscribed mailboxes are held in a json file:

  • Linux: ~/.cache/protonmail/bridge/<cacheVersion>/user_info.json (unless XDG_CACHE_HOME is set, in which case that is used as your ~)
  • macOS: ~/Library/Caches/protonmail/bridge/<cacheVersion>/user_info.json
  • Windows: %LOCALAPPDATA%\protonmail\bridge\<cacheVersion>\user_info.json

Lock file

Bridge utilises an on-disk lock to ensure only one instance is run at once. The lock file is here:

  • Linux: ~/.cache/protonmail/bridge/<cacheVersion>/bridge.lock (unless XDG_CACHE_HOME is set, in which case that is used as your ~)
  • macOS: ~/Library/Caches/protonmail/bridge/<cacheVersion>/bridge.lock
  • Windows: %LOCALAPPDATA%\protonmail\bridge\<cacheVersion>\bridge.lock

TLS Certificate and Key

When bridge first starts, it generates a unique TLS certificate and key file at the following locations:

  • Linux: ~/.config/protonmail/bridge/{cert,key}.pem (unless XDG_CONFIG_HOME is set, in which case that is used as your ~/.config)
  • macOS: ~/Library/ApplicationSupport/protonmail/bridge/{cert,key}.pem
  • Windows: %APPDATA%\protonmail\bridge\{cert,key}.pem