What are some alternatives?
Mailpile - Mailpile is a modern, fast web-mail client with user-friendly encryption and privacy features.
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
MessageOne - MessageOne offers email management solutions such as EMS and AlertFind for archiving, e-discovery, security and notification services.
NeutronMail - Self-hosted server for the ProtonMail client.
28Hands Mail Manager - Email Management Software: Mail Manager from 28Hands.