Does using ProtonMail Bridge retain end-to-end encryption and zero-access guarantees?

1. How the Bridge integrates desktop clients with Proton’s encryption model

Proton Mail Bridge runs on the user’s local machine and acts as an intermediary that speaks IMAP/SMTP to desktop clients while talking to Proton’s servers using Proton’s encrypted protocols; Bridge performs local decryption of messages downloaded from Proton and local encryption of messages before sending them back to the servers, enabling standard clients like Outlook, Thunderbird, and Apple Mail to access Proton accounts without server-side plaintext ^{[1] [5]}.

2. The core guarantees: end-to-end encryption and zero-access maintained

Proton’s documentation and product announcements explicitly state that the Bridge “preserves end-to-end email encryption, and also zero-access encryption,” meaning stored mail remains encrypted such that Proton claims it “cannot read any of your messages” and the servers do not hold decrypted user data ^{[4] [6] [2]}. Proton’s security pages repeat that E2EE and zero-access are “at the heart” of the service and that messages in the Proton Mail mailbox are stored with zero-access encryption so “no one except you can access emails stored on our servers” ^{[7] [6]}.

3. Important caveats: non‑Proton recipients, subject lines/metadata, and server transit

The Bridge preserves Proton’s guarantees only within Proton’s encryption model; emails sent to non‑Proton recipients are not end-to-end encrypted by default and rely on TLS in transit and zero‑access protection for storage on Proton’s servers unless the sender uses Proton’s password‑protected email feature or other E2EE mechanisms ^{[3] [6]}. Proton’s own explanations also highlight that messages are encrypted in transit using TLS and then decrypted and re‑encrypted for storage under zero‑access, which implies metadata handling differences and the standard limitation that subject lines and some headers may not enjoy the same end-to-end ciphertext protections as message bodies in some flows ^{[6] [3]}.

4. Endpoint risk: local decryption means local responsibility

Because the Bridge decrypts messages locally for the desktop client, E2EE and zero‑access only prevent Proton from reading messages — they do not prevent local access on an infected or compromised machine; Proton explicitly warns that Bridge “does not protect your emails from end‑point compromise (e.g. compromised laptop)” ^[4]. In other words, the zero‑access guarantee is about Proton’s inability to read stored data on its servers, not about preventing access by malware, local administrators, or legally compelled access to a decrypted endpoint.

5. Transparency, open source, and community signals

Proton has open‑sourced client and Bridge code components and presents a narrative of zero‑access and client‑side encryption in its support and security pages, and community proposals and documentation repeatedly echo the claim that servers never hold decrypted user data ^{[8] [9] [5]}. Those materials are the basis for the company’s technical claim; independent audits and scrutiny of the exact server-side architecture and how non‑E2EE flows are handled would be the relevant next step for skeptics, but those audit outcomes are not detailed in the provided sources ^{[8] [9]}.

6. Bottom line

Using Proton Mail Bridge retains Proton’s advertised end-to-end and zero‑access guarantees for mail handled within Proton’s encryption model — Bridge performs local encryption/decryption so Proton’s servers don’t hold plaintext — but the user must accept two practical limits: messages to external (non‑Proton) recipients are not E2EE by default without extra steps, and local device compromise or client misconfiguration can expose decrypted mail despite the server-side zero‑access guarantee ^{[1] [3] [4]}.