Hey there,
I’m on the search for an alternative to Mattermost for a small institution I’m working with. Mattermost was the strongest contender for our needs, yet they changed their policy regarding self-hosted instances. The factor that killed it for us, is the hard cap on 250 registered users, as we potentially might need to commodate more than that.
Rocket.Chat has similar caps.
We found Zulip, and it seems as it might be what we are looking for, but we haven’t tested yet. Nonetheless, I wanted to address this community, as you may have another good idea?
A Matrix server is a good solution. You can disable federation if you would rather not talk to other instances and have a similar experience to Mattermost, Slack, etc.
If you dislike the Matrix protocol, an XMPP server would work exactly the same way.
NextCloud Chat could also be a viable solution for internal chat.