Mattermost

Mattermost provides agencies with a Slack-like team messaging experience that can run entirely on their own infrastructure — making it the go-to choice for agencies handling sensitive client data, operating under strict compliance requirements, or simply wanting full control over their communication data. The open-source foundation ensures no vendor lock-in, and self-hosting means messages, files, and data never leave your servers.

For agencies working with government clients, healthcare organizations, financial institutions, or any sector with strict data handling requirements, Mattermost's self-hosted deployment is often a compliance necessity rather than a preference. The platform provides channels, threads, direct messages, file sharing, and search that feels immediately familiar to anyone who has used Slack. Playbooks add structured process management for recurring workflows like incident response, client onboarding, or campaign launches. Full data export and compliance tools satisfy audit requirements that cloud-hosted tools may struggle to meet.

Compared to Slack, Mattermost sacrifices the breadth of the app marketplace and the polish of mobile apps in exchange for complete data control and no per-message history limits. Compared to Microsoft Teams, Mattermost offers a cleaner, more focused messaging experience without the Microsoft ecosystem overhead. For agencies where data sovereignty is non-negotiable, Mattermost delivers enterprise-grade team communication on your terms.