Now part of Gitlab: https://about.gitlab.com/2015/08/18/gitlab-loves-mattermost/
Interesting licensing (normally AGPL/commercial but MIT just for Gitlab); I'm interested to find out more about how this is even possible: 1) "GitLab Mattermost" is available under an MIT license as a compiled version of "Mattermost", and 2) the MIT license does not apply to the "Mattermost" source code https://gitlab.com/gitlab-org/omnibus-gitlab/issues/654
One note to add is that the standard version of Mattermost is Apache-AGPL similar to MongoDB. So any organization comfortable with MongoDB should be fine with Mattermost.
MongoDB and Mattermost have Apache for drivers, configuration and administration so you can use and deploy the binaries without restriction. If modifications are made to the core product, those are required to be shared with the open source community.