Privacy and security is an important thing to have these days, not that it wasn't prior to the last five years, but it seems to be more of a necessity now than before. As such, plenty of companies are looking to provide the utmost in privacy and security for their consumers and one way this is being done is with encryption. For example, WhatsApp offers end-to-end encryption for their app. ProtonMail is another example and is an encrypted email application which offers users the peace of mind in knowing that all of the emails they send out are encrypted and that only they and the recipient are able to view the contents.
ProtonMail is still sort of a brand-new app, although it had launched earlier this year. Since the emails are encrypted, ProtonMail makes sure that all of the information you send out stays private, which means third-parties aren't capable of getting their hands on any data within them, nor are they able to intercept the email. Of course, encryption is also end-to-end. Aside from the emails sending out encrypted, the app has a feature built-in called Zero Access which means that all email data is stored in encrypted format which locks out even ProtonMail from being able to read them. The app is also open source so that developers and security researchers (security experts) are able to inspect the security protocols and the level of privacy it offers.
While ProtonMail does have the ability to sign up for a premium account, the app is free to install and free to use so you aren't required to pay. Donating to upgrade to premium simply allows the developers to continue keeping ProtonMail a free app with an open source status. ProtonMail also understands that not all the people you may be in contact with via email may be ProtonMail users or have email accounts with other services that offer encryption, so you can send those particular emails with a password protection to keep the privacy intact. This is of course is less secure than both sender and the recipient having encrypted email accounts, but it's nice for users to have the options as a backup privacy standard.