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).
Mailpile - Mailpile is a modern, fast web-mail client with user-friendly encryption and privacy features.
CleanEmail - CleanEmail is an online service that empowers you to take control of your mailbox.
Rainloop - RainLoop is a web based email client.
MailClark - The Slack bot for external communications
iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).