Backchannel Chat - Backchannel Chat is best in class discussion tool that is intended for teachers to streamline their class activities.
Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
NowComment - NowComment is a complete suite of collaboration tools, streamlining group discussion, annotation, and curation of texts, images, and videos.
Microsoft Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.
Eduflow - Digital learning that actually teaches students something
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.