Team chat tools compared: Slack, Microsoft Teams, Mattermost and Rocket.Chat

In many companies, team chat has replaced internal email. That means customer data, HR questions and supplier discussions end up there too. Choosing a tool is therefore a data protection decision. We compare four widely used options: two US cloud services and two open source platforms you can host yourself.

Cloud or your own server?

The key question is the operating model. Slack and Microsoft Teams run only as cloud services. You need no in-house IT, but your data goes to a US vendor. Mattermost and Rocket.Chat run on your own servers or with a hosting provider of your choice. You decide the location, but you are responsible for updates and security.

For cloud services you need a data processing agreement and a legal basis for transfers to the US. Slack, Microsoft and Mattermost were listed as active on the EU-US Data Privacy Framework List when we checked on 13 November 2026. Slack is covered as an entity of Salesforce. We found no entry for Rocket.Chat. The vendor uses standard contractual clauses in its DPA.

The four tools at a glance

Information as of November 2026, based on the vendors' pricing, help and trust pages. We did not run the tools in long-term operation. The evidence column reflects what the vendors publish themselves.

How do the cloud services differ?

Slack stands out for its app directory and usability, Teams for its Microsoft integration. According to Slack's help centre, data residency in a region such as Frankfurt requires Business+. Even then, member profiles and memberships are excluded. The free plan shows only 90 days of message history.

With the EU Data Boundary, Microsoft has committed to storing and processing customer data of Microsoft 365 customers from the EU and EFTA within that region. Microsoft documents the exceptions. Following commitments accepted by the European Commission on 12 September 2025, Microsoft offers suites with and without Teams in the EEA (reported by heise online, among others).

Both cloud vendors publish audit evidence. On its compliance page, Slack lists ISO/IEC 27001, SOC 2 Type II and a C5 attestation from the German BSI, among others. Microsoft includes Teams in the ISO 27001 scope of Office 365. For small businesses, such evidence is often the only practical way to assess a vendor's security.

How do the self-hosted options differ?

Mattermost is the more mature platform for small and mid-sized teams, Rocket.Chat the more flexible one for federation. Mattermost offers a free Entry edition with all features but a history limit of 10,000 messages. The Professional plan covers up to 250 users with SSO and support. Details are in our Mattermost review.

Rocket.Chat's Starter plan is free for up to 50 users and already includes LDAP and SAML. The enterprise plans add Matrix federation and end-to-end encryption, with prices on request only. The vendor itself does not consider the Community edition suitable for business-critical use.

What about calls and meetings?

All four tools offer calls, but to different extents. Teams includes full meetings. Mattermost has its own Calls feature, which is due to be limited to 40 minutes in the free Entry edition. According to its documentation, Rocket.Chat integrates Jitsi and BigBlueButton, and from Starter also Zoom, Google Meet and Pexip. With the cloud services, calls run on the vendor's infrastructure.

Video conferencing has its own data protection rules, for example on recording and default settings. Our checklist on GDPR-compliant video conferencing sums them up. Also look for single sign-on in every tool. It lets you revoke access for leavers in one central place.

Which tool suits whom?

• Microsoft Teams: companies already on Microsoft 365 that want chat, meetings and files in one place.

• Slack: cloud teams with many third-party apps that are willing to pay for Business+ to get EU data residency.

• Mattermost: businesses with Linux skills or a hosting partner that want data sovereignty.

• Rocket.Chat: small teams of up to 50 users with self-hosting experience, or organisations that need federation.

Our SaaS selection checklist shows how to structure the decision. More comparisons are listed on our comparisons overview.