What are some alternatives?
NeutronMail - Self-hosted server for the ProtonMail client.
Mailpile - Mailpile is a modern, fast web-mail client with user-friendly encryption and privacy features.
Rainloop - RainLoop is a web based email client.
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
SOGo - SOGo is groupware server with a focus on scalability and open standards.