What are some alternatives?
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).
NeutronMail - Self-hosted server for the ProtonMail client.
CleanEmail - CleanEmail is an online service that empowers you to take control of your mailbox.
Roundcube - Web-based IMAP email client
MailClark - The Slack bot for external communications
Rainloop - RainLoop is a web based email client.