Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.
Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
Notion - All-in-one workspace. One tool for your whole team. Write, plan, and get organized.
Microsoft Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.
Slack - A messaging app for teams who see through the Earth!
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.