ProtonMail's encrypted email app went live for everyone a year or so ago. The company offered a free VPN service just this past June and an encrypted contacts system just before Thanksgiving of this year. Now ProtonMail is enabling mainstream email app users safely send and receive email, too.
Called ProtonMail Bridge, the application runs in the background on your computer and will encrypt and decrypt email on the fly. It integrates your ProtonMail account with any email program that supports IMAP and SMTP, like Microsoft Outlook, Apple Mail and Mozilla Thunderbird. Setup takes a bit of effort, but ProtonMail has pictorial guides to help you through it for each email app. Using Bridge with your standard email client allows you to perform full-text searches, use multiple accounts and mass backup features; something you're not able to do with the original ProtonMail itself.