Dialog Messenger - handy and feature-rich enterprise multi-device messenger available for server or cloud – Slack-like, but not Slack-limited
Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
Ripcord - A desktop chat client for Discord and Slack
Microsoft Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.
Done Hui - No need to switch between multiple pieces of software to get through the workday. CHATS: Communicate freely. CALENDAR: Know your team's availability, plan meetings. No more conflicts. TO-DOs: Stay on top of all projects. FILES: All files, one spot.
eM Client - eM Client is a fully-featured email client for Windows and macOS with a clean and easy-to-use interface. eM Client also offers features for calendars, tasks, contacts, notes, and chat.