zulip is open source (despite being developed by a company). They don't discourage you to run your own server, it's fully documented (and we do it in our compan

zulip is open source (despite being developed by a company). They don't discourage you to run your own server, it's fully documented (and we do it in our company) It's not intended to be a Slack clone, but better by introducing email-like discussion topics. I like it, but seems that some less geeky people are more difficult to educate how to use it reasonably.