ProtonMail Bridge — Secure Gateway for Encrypted Email
General Information
ProtonMail Bridge is a desktop application created to link Proton Mail’s encrypted platform with common email clients. By default, Proton Mail keeps users inside its own web and mobile apps, but many professionals still prefer Outlook, Thunderbird, or Apple Mail. Bridge acts as the translator, handling encryption and decryption locally while exposing a standard IMAP/SMTP interface.
It’s mainly used by people who want Proton’s strong privacy guarantees but also need the convenience of a traditional desktop client.
How It Works
Bridge runs in the background on the user’s machine. It logs into Proton Mail securely, downloads encrypted messages, decrypts them locally, and then serves them to the mail client through IMAP. Outgoing messages are taken from the client, encrypted again, and sent via Proton Mail’s servers.
All cryptographic work happens locally. Proton never has access to the decrypted data, and the user keeps the familiar workflow of their mail client.
Functions
| Feature | In practice |
| Platforms | Windows, macOS, Linux (GUI and CLI versions) |
| Protocols | Provides IMAP/SMTP endpoints for clients |
| Encryption | Decrypts incoming, encrypts outgoing mail locally |
| Compatibility | Works with Outlook, Thunderbird, Apple Mail, etc. |
| Caching | Keeps local cache for offline use and fast searching |
| Authentication | Proton credentials with 2FA support |
| Security | Encryption handled entirely on the local machine |
| License | Closed-source utility, included with Proton Mail paid plans |
Installation Guide
1. Install ProtonMail Bridge on Windows, macOS, or Linux.
2. Sign in with a Proton Mail account (requires paid plan).
3. The app generates IMAP/SMTP login details for the chosen mail client.
4. Configure Outlook, Thunderbird, or Apple Mail with those details.
5. Test sending and receiving to verify encryption is working.
Linux also provides a CLI version for headless setups or scripts.
Everyday Use
– Companies use Bridge so staff can keep Outlook but still have Proton’s encryption.
– Freelancers run it with Thunderbird for productivity add-ons.
– Travelers like the offline cache to read and search mail on the road.
– Privacy enthusiasts get Proton’s zero-access model while keeping their preferred client.
For the user, it feels normal — open the client and mail works. Bridge just does the crypto behind the scenes.
Limitations
– Available only to Proton Mail paid accounts.
– Requires Bridge to run in the background at all times.
– Local cache can consume a lot of disk space with large inboxes.
– Doesn’t expose Proton-only features like Calendar or Drive.
Comparison
| Tool | Platforms | Strengths | Best Fit |
| ProtonMail Bridge | Win/macOS/Linux | Proton encryption in desktop clients | Users combining Proton with Outlook/Thunderbird |
| Proton Mail Web | Web/mobile | Zero setup, integrated E2E encryption | Users fine with webmail only |
| GPG + Standard IMAP | Cross-platform | Custom encryption with any provider | DIY users comfortable with manual setup |
| Tutanota Desktop | Win/macOS/Linux | Integrated encrypted client | Teams that can adopt a new client |
| Fastmail | SaaS | Fast, simple IMAP/SMTP, less focus on encryption | Users prioritizing speed and simplicity |
Notes from the Field
Admins often mention that the main challenge is reminding users Bridge has to stay running; otherwise Outlook or Thunderbird simply stop syncing. On Linux, the CLI version is handy for scripts, but it requires some manual setup. The cache is fast but should be secured with disk encryption if the laptop is shared. Once configured, Bridge is usually invisible — users send and receive mail as normal, unaware of the crypto operations happening locally.