Mattermost
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![](https://www.directory.trade-free.org/wp-content/uploads/2022/06/mattermost.jpeg)
Mattermost is an open-source, self-hostable online chat service with file sharing, search, and integrations. It is designed as an internal chat for organisations and companies, and mostly markets itself as an open-source alternative to Slack and Microsoft Teams.
On the website there are different models/packages on sale: https://mattermost.com/pricing/
However the software is open-source and can be hosted on your own server, for example on a Debian system: https://docs.mattermost.com/install/install-debian.html?src=dl – therefore this software can be considered as trade-free.