Mattermost
Open-source alternative to Microsoft Teams, Slack, Guilded.
Mattermost provides a flexible, open-source platform for secure team collaboration, designed for organizations with strict security and privacy requirements.
Open Source Alternative to:

Mattermost provides a flexible, open-source platform for secure team collaboration, designed for organizations with strict security and privacy requirements.
Some key features of Mattermost:
- Open-source alternative to Microsoft Teams, Slack, Guilded
- Collaboration & Communication and Data Security & Privacy coverage
- GitHub stars, forks, license, last commit, and activity score synced from directory sources
- Codex course and interactive classroom previews attached to the repository detail page
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Codex HTML Course
Mattermost Codex HTML Course
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Mattermost Codex HTML Course is being prepared for this directory project.
Interactive Classroom
Mattermost Interactive Classroom
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Mattermost Interactive Classroom is being prepared for this directory project.
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