Hi @Matter,
Our Mattermost mobile and desktop apps are just clients that allow you to access your Mattermost server that stores and manages your messaging data. You can also access your mattermost server and teams through any web browser at your server URL, for example: http://example-server.mattermost.com or http://example-server.mattermost.com/example-team. It currently isn't possible to message a Slack user from a Mattermost server, as we are two different messaging services.