Dialog Messenger - handy and feature-rich enterprise multi-device messenger available for server or cloud – Slack-like, but not Slack-limited

Google Groups - Google Groups allows you to create and participate in online forums and email-based groups with a rich experience for community conversations.

Ripcord - A desktop chat client for Discord and Slack

Groups.io - Email and web based discussion group software, includes archiving, emails, wiki, polls, file...

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.

Gaggle Mail - Gaggle Mail is a simple and easy to use email list manager.