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Mattermost is really nice, and is self-hosted. If you run a self-hosted version of GitLab as well, the GitLab omnibus may be interesting, because it contains Mattermost as well.

https://docs.gitlab.com/omnibus/

Indeed this makes upgrading the whole enchilada a breeze.

Watch out though, the MM database is not backed up by the GitLab backup scripts.

The script I wrote: https://gitlab.com/gitlab-org/omnibus-gitlab/issues/2493

The general issue tracking vendored software backup: https://gitlab.com/gitlab-org/omnibus-gitlab/issues/1927

MS Teams has decent video but everything else about is hugely inferior to Slack in my experience. Very frustrating.

I've also hosted a few mattermost instances, highly recommended. It's a YC company too which is cool.

I like that you can choose to use the smartphone app, desktop app or the web app - just like slack except you host it.