[compuguy]

I've managed a rocket.chat server at a previous job, and its very stable as well. The big plus is that its completely free (including ldap support) vs slack and mattermost.

[leipert]

Mattermost is open source as well.

If you use gitlab (CE or EE), you can easily setup Mattermost with LDAP as well: https://gitlab.com/gitlab-org/gitlab-mattermost

[compuguy]

I'm aware. But the important features for a small company (external auth, ldap) are on a paid tier for Mattermost. Its good that mattermost can tie into gitlab's authentication via ldap (no ldap group permissions though with the OSS tier).

Edit: Gitlab doesn't officially support SSO on top of SSO (AD/LDAP, etc), so its at your own peril: https://docs.mattermost.com/deployment/sso-gitlab.html