I think I’ve found a design flaw in how Proton handles password-protected emails, and I’d like to know if others see the same thing.

What happens:

1. You send a password-protected email to a non-Proton recipient.
2. They open it through the secure link and reply through the portal.
3. That reply lands in your inbox like any normal message.
4. Later, you hit Reply the way you would for any email. Nothing tells you the thread started out protected, and the reply goes out as a standard unencrypted email.
5. The recipient’s regular inbox now holds your reply, with the original protected message quoted in the thread history.

Why I think it’s a real problem:

The only visual cue is a small blue lock that appears when you set a password. When you don’t, nothing appears, and a missing icon is easy to overlook. Someone less technical could easily assume that a conversation that started securely stays secure. And since the sensitive content sits in the quoted history, even a harmless reply like “sounds good, thanks” re-sends it in the clear.

The people most likely to use this feature are the ones sending to non-Proton contacts who don’t use PGP, who are often the least likely to notice.

What I’d suggest:

• Default to re-applying password protection when replying to a thread that began as a protected message
• Show a clear warning when sending an unprotected reply to such a thread
• Strip or collapse the quoted protected content when the reply isn’t protected
• Remember the password and expiry per conversation so re-enabling it is one click

Security state should fail loudly, not silently.

I’ve reported this to Proton’s security team and filed a bug report. I reproduced it on the website using Brave Origin on Linux, October 2026 and haven’t tested the mobile apps.

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