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In my project we are evaluating Atlassian HipChat server but it is very expensive once you pass the 10 users. I need an alternative that meets as many as possible of following criteria:
Maybe you are looking for Mattermost. You can download it and install on your own server, e.g. via Docker for Windows. It has an API and you can use compiled versions under MIT license
You also can look at 4 open source alternatives to Slack for team chat.
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