Mattermost vs Slack

Mattermost

Channels, real threaded discussions and link and file previews are on the plan page, and the API docs show threads you can pull and move between channels — the move is marked beta, but it's the tidying-without-losing-history capability I ask for. Search is described as core search up to three million posts on Professional and enterprise search above that. I found no public information on end-user search filters by person or date, pinned or saved items, or how guest-channel boundaries are drawn, which keeps it off the upper band.

Slack

Channels, Slack Connect, file sharing and search across the company's entire stored knowledge are evidenced, and history is unlimited from Pro upward — the daily surface works. But nothing in the evidence speaks to threads staying readable, edit history, pinned items, filtered search, or archiving a channel without losing its history, so the 'archive as knowledge base' half of the scale is unevidenced.

Mattermost

Transport and at-rest encryption are documented down to the mechanics — TLS with named ciphers, disk-level encryption via LUKS, BitLocker or TDE, S3-managed keys — and access control runs from granular role-based permissions and AD/LDAP sync up to attribute-based zero-trust channel access. The deployment guide states plainly that it does not encrypt within the database so that search and compliance reporting keep working, which is the honest statement I want from a vendor. We found no public information on end-to-end encryption, device verification or admin-revocable sessions, so it sits mid-band.

Slack

Encryption at rest and in transit with vendor-held keys, SSO down to the free tier, 2FA, and an Enterprise Key Management add-on for customer-held keys — solid but conventional. No end-to-end encryption anywhere, no device verification or admin-revocable session management documented, and no plain statement of what Slack itself can read beyond promises about LLM training.

Mattermost

The compliance export machinery is genuinely deep — CSV, Actiance XML, Global Relay EML and Proofpoint formats, daily scheduled jobs, filters by date, user and keyword, and messages tracked by identifier as they are edited or deleted across export batches — with legal hold and eDiscovery automation named on the Enterprise plans. Default retention keeps all messages including edits and deletes, with customisable retention depending on plan. Playbooks and Boards are excluded from compliance export, and we found no public information on a full administrative audit trail or an organisation-wide switch for activity analytics.

Slack

Retention configurable at workspace and channel level (plan-dependent), plus legal hold, audit logs, DLP and information barriers on the pricing page — the legal object is at least named. Missing: eDiscovery export including edits and deletions, evidenced policy execution, and any word on whether activity analytics can be switched off organisation-wide, which is exactly what a works council asks first.

Mattermost

This is where they are strongest: the same server installs via Kubernetes, Linux or containers, a documented air-gap runbook with registry mirroring covers every edition down to Team Edition, and FIPS-compliant, STIG-hardened images are published. The privacy policy says the self-hosting customer controls the processing of end-user data, telemetry is opt-out, and Matrix-protocol federation is named on the Enterprise plan. I held back from the top only because the captured pages show compliance-format exports rather than a documented full workspace export and import for migration.

Slack

Cloud-only from a proprietary San Francisco vendor: no self-hosting, no open-source core, no documented export format anywhere in the evidence. The only custody-adjacent facts are Germany hosting from Business+ and Slack hosting its own LLMs in its own AWS private cloud — neither puts my archive in my hands.

Mattermost

There's a documented REST API with bearer auth and per-endpoint permissions, incoming and outgoing webhooks on every plan including the free Entry level, Slack webhook format compatibility for migration, interactive dialogs and Blocks for structured posts, and administrators can enforce webhook channel locking. GitLab notifications land through the standard webhook path, which is the busy-Tuesday test. We found no public information on slash commands, SCIM provisioning, documented rate limits or a sandbox, so it stops short of a full platform story.

Slack

An average of 43 apps per team and 1.7M weekly active apps say the ecosystem is real, automations are buildable 'with a click or by code', third-party services are controllable per workspace, and SCIM provisioning exists.

Mattermost

The contracting entity is Mattermost, Inc. in California, transfers to the United States are disclosed in the privacy policy, and the captured pages give no official subprocessor list for the vendor-managed cloud — Azure and AWS appear only as customer-chosen hosting. What rescues a European deployment is custody: self-hosting with a telemetry opt-out and customer control of end-user data, plus fully documented air-gap operation. We found no public information on EU data residency being offered, so this stays near the bottom for anyone buying the vendor's cloud.

Slack

Salesforce, Inc. of San Francisco is the vendor and contracting entity, Germany data residency exists but only from Business+ upward, and no subprocessor list is published — the classic 'EU residency offered while the entity sits outside the EU' position. The LLM stack staying inside Slack's AWS private cloud is a nice touch but it's still Slack's cloud, not mine.

Mattermost

Every named plan — Professional, Enterprise, Enterprise Advanced — routes through "Contact Sales", "Get Pricing" or "Request Quote", and no per-user figure appears anywhere in the captured pages. A free limited-use Enterprise Advanced edition exists for evaluation and FAQs cover how licences are sold and seat overage, but the compliance and eDiscovery capabilities a buyer needs sit in the sales-conversation tiers. I cannot compute an annual invoice for my headcount from anything published here.

Slack

Tier boundaries are unusually legible — SSO down to the free plan, unlimited history from Pro, Germany residency from Business+, DLP on every tier — and VAT-compliant invoices are a product feature. But no per-user figure is captured in this evidence and the compliance tier (legal hold, SCIM, audit) sits in Enterprise+ without a stated price, so I cannot compute my annual invoice from these pages alone.