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ProtonMail is a Swiss company offering a free e-mail service which features client-side encryption, where your web browser encrypts the message before it's sent to their server, ensuring that no one but the recipient will ever see it. Accounts are protected via two-factor authentication, PGP signing and encryption is fully automated and can be added to any message, and if the recipient is also a ProtonMail user you can activate a self-destruct date when the message will delete itself from the system whether it's been read or not. At long last anyone can easily enjoy the kind of security in digital communication that until now was possible only for elite hackers.
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Pros
Pro Genuine end-to-end encryption
Messages sent between two ProtonMail users are encrypted before leaving the sender and only decrypted by the recipient's browser by his password. Messages sent to an e-mail account on any other service have the option to be given the same protection with a one-time password of your choosing, with the option to give the recipient a hint so it's not necessary to explicitly inform them of it beforehand.
Pro Self-destructing e-mails guard against future security breaches
A difficult truth to life in the digital age is that few things are ever lost or truly forgotten anymore. While it's not foolproof since the recipient could print the e-mail or copy it to another format, there is great benefit to it anyway. Once your message deletes itself the only real proof of its origin disappears (the metadata stored with it on the server and its encryption), and any attempts to produce a copy of it are easily dismissed as forgeries.
Pro Available through a Tor hidden service
If you're concerned that it could be learned that you were the author of a message even without that message being exposed to anyone but the recipient, you can use the Tor network to re-route your internet traffic through several distant places before it finally arrives at ProtonMail. This effectively cloaks the location of any user of the service without much time or technical skill required. Just get download a copy of the Tor Browser and browse to this address: https://protonirockerxow.onion/
Pro Operated by the scientists at CERN
This service was developed at first by elite scientists in possession of sensitive data and the careful design of the whole system reflects their talent for understanding complex systems. The knowledge that the people operating the service are the same ones smashing atoms together to unlock the mysteries of the Universe underscores that this is a very different service from all other e-mail providers.
Pro Support for custom domains
With any paid plan (the smallest being $4/month, paid annually) you can connect up to two custom domains to the service, allowing you to further increase your security by owning the complete online address that you use with the service, not just the part before the '@'. Also included with any paid plan is the ability to create up to ten e-mail addresses within your account, either all arriving at your primary inbox or kept separate, which fosters security by limiting the number of people who know any one address and making it possible to manage a large amount of messages with only a single account.
Cons
Con End-to-end encryption requires sender and recipient to have an Protonmail account
This isn't a huge con, more of a caveat if you intend to use it for encrypted communications. Protonmail also has a feature for encrypting your message to non-Protonmail users by sending them a link to the encrypted message, which can only be decrypted with a key you set.