Mattermost

Mattermost is an open source platform for secure collaboration across the entire software development lifecycle. The job it does: replacing internal email with channels and direct messages.

It makes the most sense for every team. Alongside the main category they mention File Sharing, Information Services and Messaging. They are a mid-sized team, 51 to 200 people, which usually shapes who they work with.

Mattermost is based in Palo Alto, California and founded in 2016. Their site is mattermost.com.

Before you commit, look at search, retention policies, and external guest access. Ask for a trial on your own data, not a canned demo.

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