NeutronMail is described as 'Self-hosted server for the ProtonMail client' and is an app in the Online Services category. There are more than 10 alternatives to NeutronMail for a variety of platforms, including Online / Web-based, Self-Hosted solutions, Linux, Android and iPhone. The best alternative is ProtonMail, which is both free and Open Source. Other great apps like NeutronMail are Tutanota (Freemium, Open Source), Roundcube (Free, Open Source), Rainloop (Free, Open Source) and Mailpile (Free, Open Source).
Roundcube webmail is a browser-based multilingual IMAP client with an application-like user interface. It provides full functionality you expect from an e-mail client, including MIME support, address book, folder manipulation, message searching and spell checking.
Simple, modern & fast web-based email client. Modest system requirements, decent performance, simple installation and upgrade, no database required. All these make RainLoop Webmail a perfect choice for your email solution.
Mailpile is a modern, fast web-mail client with user-friendly encryption and privacy features. The development of Mailpile is funded by a large community of backers and all code related to the project is and will be released under an OSI approved Free Software license.