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).
Roundcube - Web-based IMAP email client
CleanEmail - CleanEmail is an online service that empowers you to take control of your mailbox.
mailcow dockerized - An open source mailserver suite.
MailClark - The Slack bot for external communications
Mailpile - Mailpile is a modern, fast web-mail client with user-friendly encryption and privacy features.