Blockchain is a logic, easy-to-use solution for verifying the identity of email addresses. That's why I like the idea of using Proton Mail's transparent email service: it uses blockchain to store your public keys to prevent email account breaches.
Proton Mail is based on end-to-end encryption, ensuring that only the intended recipient of the email can read it, which can be achieved by ensuring that the public key truly belongs to the intended recipient, thus preventing man-in-the-middle attacks, such as the NSA or Roskomnadzor, creating a forged public key and managing to deceive someone into using that fake key to encrypt data.
Putting users' public keys on the blockchain creates a record to ensure that these keys actually belong to them, so you only need to cross-reference each time another user sends an email. Therefore, the validation of public keys becomes transparent and unchangeable.
From there, all you need to do is perform a search to ensure that the public key matches the intended recipient, and if there is no match, immediately display a warning to the user. This feature will also be automatically enabled for Proton Mail users.
Currently, Proton has launched a beta version of key transparency on its own private blockchain for concept validation, but the idea is that it will be a public blockchain to benefit from a larger and fully decentralized scale, to prevent or hinder the possibility of attacks on smaller blockchains.
This feature, in addition to being a security measure, may be a bit overwhelming for the average user, as it is another step towards establishing trust in the system itself for all transactions on the blockchain, so you don't have to trust what you see, as everything you do will be reflected in a reliable manner.
Proton Mail now has over a billion users, most of whom are assured of privacy, in stark contrast to more widespread email services (such as Gmail). I've been using it for a long time when I want to protect myself from spam, and its features have made it increasingly attractive to me. If things continue to progress, I am likely to take this step.
(in Spanish, here)