Most teams pick a chat platform the same way they pick coffee in the break room: whatever was already there when they arrived. Slack became default at tech companies the same way Zoom became default for video calls—good enough, at the right moment, with enough critical mass that switching costs are prohibitive.
But some organizations cannot make that default choice. A defense contractor working with ITAR-controlled technical data cannot put that data on Slack's servers. A healthcare system operating under a specific BAA structure may not be able to route clinical communications through Microsoft's commercial cloud. A government agency may be required to deploy on infrastructure that meets FedRAMP High authorization or that sits entirely on-premise, air-gapped from the public internet.
Mattermost exists for those cases. It also happens to be a reasonable choice for organizations that simply want to control their communications data and can accept the trade-offs that come with self-hosting.
TL;DR
- ▸Mattermost: Open-source, self-hosted or cloud. Free self-hosted tier covers most use cases. Best for regulated environments, compliance-heavy industries, or organizations with data sovereignty requirements.
- ▸Slack: Best UX and integrations. Standard for tech industry. $12.50/user/month (Pro). Required for Slack Connect (cross-org collaboration).
- ▸Microsoft Teams: Included with M365. Deep SharePoint/Office integration. Best if you're already paying for M365 and your team tolerates the UI.
Pricing
| Mattermost | Slack | Microsoft Teams | |
|---|---|---|---|
| Free self-hosted | Yes (full features) | N/A | N/A |
| Free cloud | Yes (limited) | Yes (90-day history) | Yes (limited) |
| Pro / paid | $3.25/user/mo (cloud) | $12.50/user/mo | Included in M365 |
| Enterprise | $8.50/user/mo | $22.50/user/mo | M365 E3/E5 |
| Self-hosted | Yes | No | No |
| SSO included | Yes (self-hosted) | Business+ only | Yes |
The M365 variable: Teams is "free" only if you're already paying for Microsoft 365. A business on M365 Business Basic ($6/user/month) gets Teams included. A business evaluating Teams standalone is looking at $4/user/month for Teams Essentials—but without Office apps, the full M365 case usually applies.
Mattermost's self-hosted math: The free self-hosted tier is unlimited users, unlimited message history, unlimited channels. For a 50-person team, Slack Pro costs $7,500/year. Mattermost self-hosted on a $40/month server costs $480/year in infrastructure. That's $7,000/year in savings—which is enough to justify an afternoon of setup time.
Feature Comparison
| Feature | Mattermost | Slack | Teams |
|---|---|---|---|
| Message history | Unlimited (self-hosted) | 90 days (free) | Unlimited |
| Channels / spaces | Yes | Yes | Yes (Teams/channels) |
| Direct messages | Yes | Yes | Yes |
| Threads | Yes | Yes | Yes |
| File sharing | Yes | Yes | Yes (SharePoint) |
| Native audio/video | Via Jitsi plugin | Yes (Huddles) | Yes (strongest here) |
| Screen sharing | Via plugin | Yes | Yes |
| Integrations | 700+ | 2,600+ | 700+ |
| Slash commands | Yes | Yes | Yes |
| SSO / SAML | Yes (all tiers) | Business+ | Yes |
| Compliance export | Enterprise | Yes | Yes |
| Data retention policies | Yes | Yes | Yes |
| E-discovery | Enterprise | Yes (paid) | Yes (Purview) |
| Self-hosted | Yes | No | On-premise (GCC High) |
| FedRAMP authorized | Yes (cloud + self-hosted) | No | Yes (GCC High) |
| Air-gapped deployment | Yes | No | No (standard) |
| Mobile apps | Yes | Best | Yes |
Mattermost: Built for When Cloud Is Not an Option
Mattermost's product was designed around a constraint that Slack ignores: some organizations cannot send their internal communications through a commercial cloud provider's infrastructure.
ITAR (International Traffic in Arms Regulations) covers technical data related to defense articles. An engineer at a defense contractor discussing specific technical specifications may be moving controlled data. Putting that conversation on Slack's servers—servers operated by a commercial entity, subject to their privacy policies and legal obligations, creates compliance exposure. ITAR doesn't have a "but Slack has good security" exception.
HIPAA in healthcare is more nuanced. Slack offers a Business Associate Agreement (BAA), which means Slack can be used for HIPAA-covered communications if configured correctly. But some healthcare systems, particularly those with specific legacy compliance frameworks or additional state-level requirements, prefer infrastructure they operate and audit directly.
FedRAMP requirements in government procurement mandate that systems holding federal data meet specific security controls at authorization levels (Low, Moderate, High). Mattermost's cloud offering holds FedRAMP authorizations. Their self-hosted deployment can be run in FedRAMP-authorized environments like AWS GovCloud or Azure Government. Slack does not hold FedRAMP authorization.
Air-gapped environments are the clearest case: Mattermost runs with no internet connectivity required once deployed. A military installation, a nuclear facility, or an intelligence community system that operates on classified networks uses Mattermost because Slack literally cannot be deployed there.
Mattermost's compliance features in the Enterprise tier include: Compliance Export (automated export of all message data for e-discovery, in formats compatible with legal review platforms), granular data retention policies per channel, custom user roles, advanced audit logging, and integration with enterprise directory services (AD, LDAP, SAML).
The self-hosted setup: Mattermost runs on Docker Compose with Postgres. A mid-sized deployment (50–200 users) needs 4GB RAM and reasonable disk for file storage. Setup takes 1–2 hours. The admin panel handles user management, channel configuration, and integrations through a browser UI.
What self-hosting Mattermost actually means day-to-day: Someone owns the server. That means applying updates (Mattermost releases monthly), monitoring disk space as file uploads accumulate, maintaining backups of the Postgres database, and responding when the server has an issue. For regulated organizations that already have ops infrastructure, this fits naturally into existing processes. For teams without an ops function, it's a real ongoing commitment.
Slack: The Default for Good Reasons
Slack became the standard for technology companies and has maintained that position because it earns it: the product is genuinely better on UX, integrations, and cross-organizational collaboration than either alternative.
Integrations are where Slack's lead is most pronounced. 2,600+ native integrations, including deep two-way connections with GitHub, Jira, PagerDuty, Salesforce, and the rest of the modern SaaS stack. Slack's workflow builder handles no-code automation. Most SaaS products' notification systems default to Slack webhooks first.
Slack Connect is the feature neither Mattermost nor Teams matches well: shared channels between organizations. A company can work in a Slack channel with a client or vendor in real time, in a space that both organizations' teams can access, with all the Slack UX intact. For agencies, consulting firms, and companies with complex external collaboration needs, this has no equivalent.
Huddles (lightweight audio) and the AI-powered features (channel summaries, thread summaries, search) that Slack has been building since 2023 make day-to-day communication faster. The mobile app is the best in this comparison.
Where Slack loses: The free tier's 90-day message history limit is genuinely restrictive, teams lose institutional knowledge constantly. Pro at $8.75/user/month adds up: a 100-person team pays $15,000/year. SSO is locked to Business+ at $22.50/user/month, which is a real irritant for security-conscious organizations that want to enforce centralized authentication without paying the premium tier price. No self-hosting option.
Microsoft Teams: The Enterprise Default
Teams is the right answer for organizations already paying for Microsoft 365, which, for enterprises, is most of them. If you're already paying $12–36/user/month for M365, Teams is included and deeply integrated: files live in SharePoint, meetings integrate with Outlook calendar, and the compliance story runs through Microsoft Purview (eDiscovery, retention policies, communication compliance).
Teams' audio and video quality leads this comparison, better than Slack Huddles, better than Mattermost's Jitsi integration. For organizations where video meetings are the primary communication mode, this matters.
Where Teams loses: The chat UX is worse than Slack. The information architecture (Teams → Channels → Posts, with General channels and standard channels and private channels all behaving slightly differently) confuses new users. Notification defaults are aggressive. The context switching between chat and "the full Teams interface" is friction Slack doesn't have.
Teams also has self-hosted options. Microsoft GCC High for US government customers, and on-premises deployments via Microsoft Teams Rooms and Skype for Business Server migration paths, but these are complex, expensive, and not comparable to running Mattermost on a $40/month server.
The Compliance Decision Tree
You must self-host if: Your communications involve ITAR-controlled technical data, you operate in an air-gapped environment, your FedRAMP authorization level requires it, or your legal team has assessed that commercial cloud chat creates unacceptable data sovereignty risk.
Self-hosting is worth evaluating if: You're in a regulated industry with specific data residency requirements, your team size makes Slack Pro pricing painful ($15,000+/year), you want full message history without paying enterprise rates, or you want SSO included without a pricing tier jump.
Stay with Slack if: External collaboration via Slack Connect is central to your workflow, your team's toolchain is Slack-first (and switching costs are real), or you need the broadest integration ecosystem without custom development.
Default to Teams if: Your organization is already paying for M365 at scale, and the incremental cost of any other platform is hard to justify to procurement.
The Verdict
Mattermost occupies a specific and defensible position: it is the answer for organizations where cloud chat is not an option, and a reasonable answer for organizations that want to control their communications infrastructure without paying enterprise SaaS rates.
It is not the better product for most teams in most circumstances. Slack is better to use. Teams is better integrated with Microsoft's ecosystem. Mattermost is better when the question is "where does this data live and who controls it?", and for regulated industries, that question has a compliance answer, not a preference answer.