Can Thunderbird decrypt ProtonMail messages through the ProtonMail Bridge?

1. What the Bridge does in plain terms

ProtonMail Bridge is a local, open‑source middleware that sits between ProtonMail’s encrypted servers and desktop mail clients. It handles end‑to‑end encryption, key management, and on‑device decryption, then exposes a standard IMAP/SMTP interface that Thunderbird can use just like any other mail server ^{[3] [4]}.

2. How Thunderbird gets decrypted mail

Thunderbird connects to the Bridge’s local server (typically on 127.0.0.1). The Bridge decrypts messages on your machine and “sends your decrypted data to your chosen email client on a restricted communication channel that never leaves your device” — that is how Thunderbird ends up showing plaintext messages ^{[2] [1]}.

3. Security and privacy framing from Proton

Proton’s materials emphasize “zero‑access” and end‑to‑end encryption is preserved because decryption happens on the user’s device, and Proton says passwords and PGP keys never leave your machine and Bridge does not permanently store decrypted message data on disk ^{[5] [3]}. Proton positions Bridge as a way to retain ProtonMail’s privacy model while using familiar desktop clients ^{[1] [4]}.

4. Practical benefits and tradeoffs for users

Using Bridge with Thunderbird gives full‑text search, native client workflows (dragging messages between accounts, multiple accounts in one client), and the convenience of desktop features — because the Bridge supplies decrypted content locally for Thunderbird to index and display ^{[1] [6]}. The tradeoff is that decrypted data exists on your machine while in use, so local device security becomes the limiting factor; Proton’s documentation stresses the local, loopback nature of the connection but does not eliminate device risk ^{[2] [1]}.

5. Compatibility and setup realities

Proton officially supports Thunderbird, Outlook, and Apple Mail; in practice any IMAP/SMTP client that can talk to a localhost server can work with Bridge. Proton provides step‑by‑step setup guides for Thunderbird on major OSes and notes Bridge runs in the background to perform encryption/decryption transparently ^{[3] [4]}.

6. User signals and ecosystem tools

Because Thunderbird won’t label which messages were end‑to‑end encrypted when using Bridge, third‑party add‑ons exist to show encryption status inside Thunderbird for users who want visual cues ^[7]. Community and vendor blogs explain how Bridge acts as a “translator” between ProtonMail’s encrypted format and standard mail clients, underscoring the practical approach Proton took to bridge ^{[8] [9]}.

7. Limits of available reporting and remaining questions

Available sources describe how Bridge decrypts locally and exposes data to the client but do not provide deep forensic detail on ephemeral storage, exact memory handling, or all threat models; for example, Proton claims decrypted data is not permanently stored on disk but the documentation does not show independent audits of runtime memory handling in these snippets ^{[5] [2]}. Independent security assessments or auditor reports are not included in the provided results — not found in current reporting.

8. Bottom line for users deciding whether to use Thunderbird + Bridge

If your goal is to use ProtonMail within Thunderbird and see readable messages there, Bridge delivers that: it decrypts on your computer and provides decrypted mail to Thunderbird over localhost ^{[1] [2]}. If your priority is minimizing any plaintext presence on a local device, recognize that Bridge’s model necessarily places decrypted content on your machine during use, so local device security becomes the critical control ^{[2] [4]}.

Sources cited: Proton documentation and related reporting as supplied ^{[5] [3] [2] [1] [7] [4] [9] [6] [8]}.