What are some alternatives?
NeutronMail - Self-hosted server for the ProtonMail client.
Roundcube - Web-based IMAP email client
ProtonMail - Secure email with absolutely no compromises. Get your free encrypted email account today.
Rainloop - RainLoop is a web based email client.
Gmail - Gmail is a free search-based web-mail service.
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.