Install citadel-server by entering the following commands in the terminal:
sudo apt update sudo apt install citadel-server
complete and feature-rich groupware server
Citadel is a complete and feature-rich open source groupware platform. * Email, calendaring/scheduling, address books * Bulletin boards, mailing list server, instant messaging * Multiple domain support * An intuitive, attractive AJAX-style web interface . The Citadel system is extremely versatile. It provides numerous front ends to present to users, such as a text-based interface, an AJAX-style web interface, and many popular PIM clients using SMTP/POP/IMAP. All of these can be used simultaneously. . It's also extremely scalable. Not only can a well-equipped Citadel server support a large number of concurrent users, but you can also build a distributed network of Citadel nodes that share rooms and their content.