Mattermost

Our verdict: a mature, self-hostable team-chat platform with an open-source core — the strongest option when keeping your messages on your own infrastructure matters, provided you accept that some enterprise features are paid and source-available rather than fully open.

Mattermost is a self-hostable team messaging platform built around channels, threaded conversations, direct messages and file sharing, with built-in voice/video calls and Playbooks for workflow and incident-response automation. It runs as a single Linux binary backed by a PostgreSQL database, with native desktop (Windows/macOS/Linux) and mobile (iOS/Android) apps plus a web client, and is developed by Mattermost, Inc. in the United States. It is commonly used as a self-hosted alternative to Slack.

Its licensing is open-core rather than fully open source: Mattermost's compiled Team Edition binaries are MIT-licensed and self-hostable, while the underlying server source is AGPL-3.0-only (or a commercial license) and the admin tools and configuration are Apache-2.0. The same monorepo also ships Enterprise / 'source-available' components that are viewable but require a paid Enterprise license to use in production, and a managed Mattermost Cloud is offered as a paid tier. When self-hosted, your messages and files stay on your own server and database; the server sends diagnostic/usage telemetry by default, which admins can disable in settings.