Dialog Messenger - handy and feature-rich enterprise multi-device messenger available for server or cloud – Slack-like, but not Slack-limited
Polymail - Native email app for email productivity.
Ripcord - A desktop chat client for Discord and Slack
KanbanMail - A Kanban board for your emails.
Ziggs - Smoothly Share Content Between Devices!
Crugo - Crugo is a simple group collaboration platform for your team