Most teams that move away from Slack are not doing it because the product is bad. They are doing it because someone in legal, compliance, or IT looked at where the conversation data lives and decided that putting every internal discussion on a vendor's cloud servers is not acceptable. Healthcare teams have HIPAA (the Health Insurance Portability and Accountability Act, a US law requiring strict controls on how patient-related data is stored and accessed). Defense contractors have FedRAMP (a US government security framework that sets strict requirements for cloud systems handling government data). Law firms have client confidentiality. For all of them, Slack's answer of "your data lives on our servers, here is a trust page" is not enough. Mattermost is built for exactly that situation.
It is an open-source team messaging platform that you deploy on your own infrastructure. Messages, files, and user accounts never leave your servers. You get channels, direct messages, threads, file sharing, search, bots, integrations, and an AI assistant layer, all running inside your own network. The project is at github.com/mattermost/mattermost with 38,612 stars and has been in production use at organizations including the US Department of Defense, Samsung, and NASA.
| Fact | Value |
|---|---|
| What it is | Self-hosted team messaging platform |
| Built by | Mattermost, Inc. (open-source community edition) |
| License | MIT (Team Edition) |
| Platforms | Linux, Docker, Kubernetes; clients for iOS, Android, desktop, web |
| Requires | A Linux server; at least 1 GB RAM for small teams |
| Install method | Docker or Linux package installer |
| Verdict | Install if your team handles sensitive communications and cannot use a cloud messaging service |
What Mattermost actually is
Mattermost is a web application that runs on a server you control and provides a messaging interface that your team accesses through a browser or a desktop or mobile client. The server stores all messages in a PostgreSQL database (a widely used open-source relational database, meaning it stores data in structured tables) that sits on the same infrastructure you manage.
The Team Edition is the free, MIT-licensed (a permissive open-source license that lets you use and modify the software without restrictions) tier. It covers most of what a team under 100 people actually needs: unlimited channels, direct messages, group messages, threaded replies, file uploads, search across message history, webhooks (a way for external software to send messages into Mattermost automatically when something happens), slash commands (text commands that trigger actions, typed directly in the message box), and a basic permissions system. There is an Enterprise Edition with SSO (Single Sign-On, a system where your team logs in with an existing company account rather than a separate Mattermost password), SAML authentication (Security Assertion Markup Language, a standard protocol that enterprise identity systems use to verify who a user is), and advanced compliance tooling if you need it, but the free tier is genuinely useful on its own.
The architecture is a single server binary backed by a PostgreSQL database and an optional file storage layer. Unlike tools that require a cluster of services to operate, a small Mattermost install is straightforward to run and maintain. Larger organizations can deploy it on Kubernetes (an orchestration system that manages containers across a cluster of servers, used when a single server is not enough for reliability or scale). The default single-server configuration handles up to 10,000 registered users on a mid-tier server.
What you get after installing it
Once Mattermost is running, the interface is deliberately familiar to anyone who has used Slack. You get a left sidebar with channels organized by category, a message composition area, threaded replies, emoji reactions, and message formatting. The learning curve for a team migrating from Slack is close to zero.
Channels are either public (visible to anyone in your team) or private (invite-only). There is no cap on the number of channels in the Team Edition. Direct messages work one-to-one and in groups. Threads let conversations branch off a parent message so the main channel does not fill up with back-and-forth on a single topic.
File sharing and search are included. You can upload files directly into messages, and Mattermost indexes message text so you can search conversation history. On self-hosted installs, search uses the database directly; larger deployments can add Elasticsearch (a search engine that handles full-text search across millions of records efficiently) for faster results at scale.
Integrations connect Mattermost to your existing tools. The incoming webhook format (a URL that accepts JSON payloads, where JSON is a structured text format that software uses to send data, and posts them as messages) is compatible with tools that already send Slack notifications, so migrating notification pipelines is often just swapping a URL. Outgoing webhooks (a feature where Mattermost sends a POST request, meaning an HTTP network call that delivers data, to an external URL every time a message matches certain criteria) let you build bots that respond to team messages. There is also a plugin system that lets developers extend the server-side and the client interface.
Playbooks is a workflow automation tool built into Mattermost. A playbook (a pre-defined sequence of tasks assigned to people) lets teams run structured processes like incident response or on-call handoffs directly inside the messaging interface. When a trigger fires, Mattermost creates a dedicated channel, assigns tasks, and tracks completion status.
Mattermost Copilot is the AI layer. It connects to a self-hosted LLM (Large Language Model, the type of AI model that powers tools like ChatGPT) via an API. The design matters here: because Mattermost runs on your servers and the LLM connection is also configured by you, conversation data sent to the AI layer does not pass through any external cloud service unless you configure it that way. Teams that need an AI assistant but cannot send data to OpenAI or Anthropic can run a local model like Llama or Mistral and connect Copilot to that.
Compliance features include an audit log (a tamper-evident record of every administrative action), configurable data retention policies (automatic deletion of messages older than a set date), and e-discovery export (a way to export specific message history in a format admissible in legal proceedings). These are the features that make Mattermost worth the operational overhead for regulated industries.
The install experience
The fastest path to a running Mattermost instance is Docker (a tool that packages software into containers, which are isolated runtime environments that include all dependencies). The official Docker Compose file (a configuration format that defines multiple containers and how they connect) sets up the Mattermost server, a PostgreSQL database, and a local file storage volume together.
git clone https://github.com/mattermost/docker
cd docker
cp env.example .env
docker compose up -d
After those four commands, Mattermost is running at http://your-server-ip:8065. The setup wizard asks you to create an admin account, name your workspace, and invite your first teammates. The whole process from zero to a working messaging server takes under 30 minutes, including reading the documentation.
The alternative is the Linux package installer. Mattermost publishes .deb packages (the native software package format for Ubuntu and Debian, installable via the apt command) for Ubuntu and Debian and .rpm packages (the equivalent format for Red Hat-based distributions like AlmaLinux and Rocky Linux, installable via dnf or yum) for those systems. This path gives you more control over the database and file storage configuration, and it integrates better with system-level tools like systemd (the process manager used by modern Linux distributions to start and stop services). For production deployments where you want Mattermost to restart automatically if the server reboots, the package install is the cleaner approach.
The one legitimate friction point is the reverse proxy configuration. A reverse proxy is a web server that sits in front of your application, receives incoming traffic, and forwards it to the right service while handling SSL and domain routing. Mattermost on port 8065 is fine for internal testing, but production deployments need a reverse proxy in front of it to handle SSL termination (the process of accepting HTTPS connections and passing them to the application as plain HTTP) and to serve Mattermost at a standard HTTPS address. The documentation covers Nginx and Apache configurations, and the steps are not difficult if you have set up a reverse proxy before. If you have not, plan an extra hour.
Where it fits and what to compare it to
Mattermost targets teams where data residency (the requirement that data stays within a specific geographic or organizational boundary) is a hard constraint, not a preference. If your contracts, regulations, or security policy say that communications cannot transit or rest on a third-party cloud, Mattermost is one of the few messaging platforms that can satisfy that requirement while still providing a modern interface.
Slack is the most common comparison. Slack at the Pro tier costs $7.25 per user per month and is cloud-only, meaning all message data lives on Salesforce-owned infrastructure. Slack's compliance features are strong for most commercial use cases but are not sufficient for teams that need full data residency. Slack wins on integrations breadth and polish; Mattermost wins when the data has to stay local.
Microsoft Teams comes bundled with Microsoft 365 plans and is the default choice at organizations already deep in the Microsoft stack. It is cloud-hosted in the Microsoft Azure infrastructure, and data residency controls are available at higher Microsoft 365 licensing tiers. Teams' integration with SharePoint and Outlook is hard to match. The tradeoff is dependency on Microsoft licensing and the complexity of the Teams administration model. Mattermost is simpler to administer and does not require a Microsoft account for every user.
Rocket.Chat is the other major self-hosted open-source messaging platform in the same category. It has a broader feature set than Mattermost out of the box (video conferencing, livechat for customer-facing use), a larger plugin marketplace, and a MongoDB-backed architecture (MongoDB is a document database that stores data as flexible JSON-style records rather than in fixed tables) rather than PostgreSQL. Rocket.Chat's free tier has historically had slower release cycles and more reported stability issues in community forums. Mattermost has a more focused feature scope and a stronger reputation in defense and government deployments. For a team that specifically needs a compliance posture, Mattermost is the more mature choice.
Verdict
Install Mattermost if your team cannot use a cloud messaging platform and you need something that a non-technical team member can actually navigate. The combination of familiar Slack-like interface, self-hosted data storage, robust compliance tooling, and a bot-friendly API covers the main reasons security-conscious teams stay on outdated internal tools or cobble together email threads and shared drives.
Hold off if your team has no compliance constraint and just wants a Slack alternative because the free Slack tier has message history limits. For that problem, the operational overhead of running your own messaging server is not worth it. The free Slack tier or a hosted alternative like Discord is the right tradeoff there.
The AI agent integration case is real. Mattermost's incoming webhooks, outgoing webhooks, and bot accounts give any AI agent a clean channel into team communication. An agent running on your infrastructure can post status updates, accept commands from team members, and trigger workflows without those messages transiting an external cloud service. For teams building internal agent workflows where the agent's input and output involve sensitive business data, keeping the communication layer on-premises is the same discipline as keeping the agent itself on-premises. Mattermost closes that loop cleanly.
The project has been in active development since 2015 and the mattermost.com documentation is detailed and current. The 38,612 GitHub stars reflect a decade of production use in regulated environments, not just developer interest. If your team has the compliance requirement, this is the project to evaluate first.