I'm not the parent-level poster but I've stood-up a Mattermost instance a few times and it's really easy to get going and is good for a text-IM/DM channel/group service. The desktop app or web-based interface work quite well and the architecture is pretty sane, Javascript front-end, golang-based "backend", Postgres database.

But, there are some frustrating aspects.

LDAP is only available in the "enterprise" edition which is kind of crazy and there is no price-break for < 10 users. So for personal / non-commercial usage if you want LDAP you're placed into an enterprise bucket. I reached out to Mattermost and pointed this out and even said "Hey, what about offering a 10-user license for some reasonable fee?" No response.