TL;DR: Proton Mail introduces a new blockchain-based feature called Key Transparency. It uses blockchain, but not for cryptocurrency purposes. Instead, it verifies email addresses’ ownership by putting users’ public keys on a blockchain, ensuring they’re authentic and unchanged. This feature aims to prevent “man-in-the-middle” attacks in email communication and will be automatic for Proton users, giving a warning if a public key doesn’t match its intended recipient. Currently, it’s on a private blockchain, with potential future plans for a public one. It’s designed for users needing secure email communication, like world leaders or activists, and responds to increasing privacy concerns in mainstream email services.

Eli5: Imagine you’re sending a secret letter to your friend, but you want to make sure no one else can read it or pretend to be your friend. Proton Mail has made a special way to check if your email really goes to your friend and not someone else. They use a special list (called blockchain) where they keep a special code for each person. When you send an email, they check this list to make sure the email goes to the right person with the right code. It’s like having a secret handshake only you and your friend know, so you’re sure you’re talking to each other and not someone pretending to be your friend.