Mattermost is a self-hosted, open source, private cloud-based messaging platform designed for teams and organizations. It is designed to provide an easy-to-use and secure collaboration platform for organizations of any size. It provides rich features such as real-time messaging, video conferencing, searchable chat history, file sharing, and integrations with external applications. With Mattermost, teams can communicate and collaborate on projects and tasks, in a secure and private environment. The platform is available as both a mobile and web app, with support for multiple languages. It also provides powerful collaboration features, such as the ability to share and comment on files, and to create private channels. Mattermost also provides enterprise-grade security and compliance features, such as single sign-on (SSO), audit logging, and data encryption.
Telegram is just a simple chat. Mattermost has task management, calendar and other important features.