Mail Checker for Proton Mail

No. As of 2026, Proton offers no official browser extension for Proton Mail — its Mail clients are the web app, the mobile and desktop apps, and Proton Mail Bridge (IMAP/SMTP for desktop email clients, on paid plans). Proton Pass and Proton VPN do have official extensions, but those aren’t for Mail. Mail Checker for Proton Mail is an independent extension and is not affiliated with Proton AG.

Mail Checker lives in your browser toolbar. It signs in to your account directly using Proton’s SRP login and checks for new mail in the background, so you can read, reply and compose from the popup without opening mail.proton.me in a tab.

Proton’s web app shows new-mail desktop notifications while Proton Mail is open in a browser tab (per Proton’s support docs); for other cases Proton points to its mobile apps and email notifications. Mail Checker is built for exactly this gap — it polls in the background on an interval you choose and shows desktop notifications without a Proton Mail tab open. More on Proton Mail notifications in the browser →

Yes. Mail Checker monitors several Proton Mail accounts at the same time, each with its own folders, labels, colour and notification settings. (The free tier covers a single account; multiple accounts are part of Pro.)

It signs in with Proton’s SRP protocol, so your password never leaves your device. Its browser host access is limited to *.proton.me and it talks only to Proton’s servers — the only exceptions are hCaptcha during Proton’s human-verification step and a small licence check for Pro subscriptions. There is no analytics and no telemetry. The Data & Encryption section above lists exactly what is stored and where.

Yes, and you don’t have to take our word for it. Any software you type a password into, including Proton’s own apps, holds it in memory for that moment, and that is unavoidable. The question that matters is whether it sends or stores your password anywhere it should not, and both are things you can check from outside the extension. Sign-in uses Proton’s SRP protocol, so your password is never transmitted, not even to Proton: a proof is computed on your device and only that proof is sent. The extension also verifies Proton’s signature on the login parameters and checks the server’s counter-proof, so a tampered login exchange is detected and aborted. Your password is used only on your device, to build that proof and to unlock your Proton keys, and two-factor codes go only to Proton.

Between sessions the extension keeps only the session tokens and the derived key passphrase it needs, encrypted with AES-256-GCM — the Data & Encryption section above lists exactly what is stored, where, and the honest limits of that encryption. With default settings all traffic goes to Proton’s own servers, with two scoped exceptions: Proton’s human-verification step (which loads Proton’s hCaptcha page), and a licence check for paid subscriptions that receives your Paddle subscription ID and nothing else — without a licence key the extension never contacts our servers at all. There are no analytics, no telemetry and no crash reporting of any kind (the Firefox manifest formally declares “no data collection”), and remote content in emails stays blocked until you choose to load it.

To check all of this yourself: every release ships as readable, unminified source. The Firefox package is the source verbatim — no bundler, no minifier — so you can download the .xpi from the add-on listing, unzip it, and read exactly the code that runs; the only minified files are the two official cryptography libraries (OpenPGP.js and bcrypt.js), and every listed release additionally goes through Mozilla’s add-on review. There is no certificate pinning and no traffic obfuscation, so you can watch every request the extension makes in your browser’s own devtools. Mail Checker is an independent project and is not affiliated with Proton AG — if you would rather trust no third party at all, Proton’s own web app is the right choice.

Proton Mail Bridge is a desktop app — available on paid Proton Mail plans — that runs a local IMAP/SMTP server so you can use a separate email client such as Thunderbird or Outlook. Mail Checker is a browser extension with its own popup, needs no separate client, and has a free tier. They solve different problems.