If your organization wants to use Mattermost but does not want a custom installation, Relay (https://relay-chat.com) is a cloud-hosted Mattermost installation, ready out of the box.
We contribute all our work on Relay back to the Mattermost core and Relay itself will always be open source.
We will help you manage Slack imports, Slack bridging, integrations, and plugins.
Try Relay by connecting your Mattermost client to our open server on https://open.relay-chat.com
Let us know if you have any questions!