What is ProtonMail, the service used by Cambridge Analytica to cover its tracks?

If you've been out of the loop of the Facebook/privacy/Cambridge Analytica scandal that has ruled headlines over the past week, there's a small yet incredibly important detail you may have missed. 

Cambridge Analytica — the data analytics firm that came under fire this weekend for maliciously collecting information on 50 million Facebook users — reportedly used a self-destructing, encrypted email service called ProtonMail to cover its tracks, covering up correspondence between the company and third parties, according to a Channel 4 News investigation published Wednesday. The firm set emails to self-delete after two hours and urged clients to use the service as well, per footage captured of former CEO Alexander Nix talking to a journalist posing as a would-be client. Read more...