We use Zulip which is an unfortunate mix of awesome and mediocre. Like Mattermost Zulip can be self-hosted, although we use the online paid for version. Zulip

We use Zulip which is an unfortunate mix of awesome and mediocre. Like Mattermost Zulip can be self-hosted, although we use the online paid for version. Zulip's USP is the 'topic' system which group messages in each thread into a coherent conversation. It makes it much easier to look back over a set of conversations and see what happened. It works really well. It's let down by the mobile apps though, which are clunky and don't behave reliably. The web-app is excellent though and the topic system does (just) win out over the crap apps. I do wish they'd sort them out though, both for me and so I could make an unqualified recommendation.