When using email, many people assume their emails remain confidential between the sender and receiver. What is often overlooked when sending an email is how it reaches its destination, and the form it takes when being transmitted. Although privacy is desirable, few people will go to the extent required to ensure their personal data remains confidential. Although end-to-end encrypted email solutions have been available for many years to provide confidentiality of email correspondence between sender and receiver, it has yet to see wide adoption by the general public. ProtonMail is a secure email application that supports end-to-end encrypted email. Using a cognitive walkthrough as the primary assessment tool, this thesis assesses the usability of ProtonMail to determine if it addresses the challenges of end-to-end encrypted email found when using S/MIME or PGP as an end-to-end encrypted email solution. The results of the assessment indicate that although ProtonMail provides an effective solution for sending end-to-end encrypted emails between ProtonMail users, it is ineffective when sending end-to-end encrypted emails outside of ProtonMail. It is suggested that although ProtonMail provides an adequate solution for sending encrypted email between ProtonMail users, it has yet to solve the challenges that are associated with public-key distribution found when using S/MIME and PGP in a public-setting.