What are some alternatives?
NeutronMail - Self-hosted server for the ProtonMail client.
Astro - The ASTRO Command Center software is specifically intended for use with: MixAmp Pro TR (late 2015 release), A50 Gen 3 with Base Stations (late 2016 release), A20 Wireless (late 2017 release).
Roundcube - Web-based IMAP email client
Hiver - Manage Customer Support, Sales and Projects right from your Gmail inbox. Collaborate with Shared Labels, Shared Contacts, email notes, email reminders and email templates
Rainloop - RainLoop is a web based email client.
CleanEmail - CleanEmail is an online service that empowers you to take control of your mailbox.