Mattermost vs Slack

Mattermost

Channel-based collaboration, threaded discussions, link and file previews, guest accounts and search up to three million posts are documented, and the thread structure is exposed through the API with permissions and pagination detail. Compliance exports flag edits and deletions per message, which tells me the history beneath the surface stays intact. We found no public information on pinned and saved items, on end-user search filters by channel, person or date, or on channel lifecycle such as archive and rename without losing history.

Slack

Channels and Slack Connect with customer-controlled external boundaries, enterprise and AI-assisted search, and documented history limits (1 year on free, unlimited on paid) give a solid daily surface. But the evidence is silent on threads, message-editing history, and — the part I care about — any export of a conversation in a form a human or a court can read.

Mattermost

TLS with named ciphers, encryption-at-rest through customer-side disk and database methods, granular role-based access controls, SSO with SAML assertion encryption and attribute-based zero-trust channel access at the top tier are all documented. The vendor states plainly that the database itself holds no encryption so that search and compliance reporting work — candour I value, though it confirms the server sees plaintext. We found no public information on end-to-end encryption, device verification, or session management an administrator can revoke.

Slack

Encryption at rest and in transit, SAML SSO, 2FA, SCIM, information barriers and EMM plus customer-controlled posting permissions, DM visibility and Slack Connect gating clear the anchor-5 bar, and the Enterprise Key Management add-on at least puts key handling on paper. But there is no end-to-end encryption anywhere in the evidence, no device verification or session revocation, and no plain statement of what the vendor itself can read.

Mattermost

The compliance export is the real thing: CSV, Actiance XML, Global Relay EML and Proofpoint formats, edits and deletions flagged per message, message tracking across export batches, channel member history, past-history export from a timestamp, and query and download actions logged in an audit history. Legal hold and eDiscovery automation are named on the Enterprise plan, and the default is to retain everything including edits and deletes. Two reservations hold this back: Playbooks and Boards content is documented as excluded from compliance export, so decisions that migrate there escape the subpoena, and we found no public information on per-channel retention granularity or organisation-wide switching of activity analytics.

Slack

Retention configurable at workspace and channel level, plus audit logs and a named legal-hold feature, carry this past rubric level 3. What the evidence never evidences is exactly what I would be subpoenaed for: an eDiscovery export including edits and deletions, proof that retention deletions are actually executed, and any organisation-wide switch for activity analytics — named bullets on a pricing page are not a discovery process.

Mattermost

Custody can be entirely the customer's: the same server installs via Kubernetes, Linux or containers, high availability is self-managed, FIPS-compliant STIG-hardened images are published, and the air-gap documentation includes a runbook, bill of materials and registry mirroring. The privacy policy states that with self-hosted products the customer controls the processing of end user data, and federation via Matrix protocol interoperability is named on the Enterprise plan. We found no public information on a documented migration path in and out, and the source availability of the server is not confirmed on the captured pages.

Slack

Nothing in the evidence offers the customer custody: no self-hosting, no private instance, no open source, and — decisive for an exit — no documented export of any kind. The EKM add-on and Germany hosting from Business+ are controls over Slack's cloud, not an alternative to it, so this sits at the bottom of the scale nudged up only by key custody.

Mattermost

A documented REST API with bearer authentication and per-endpoint permission detail, incoming and outgoing webhooks with Slack-format compatibility, interactive components via blocks and dialogs, and SSO with AD/LDAP sync are all evidenced. Administrators can enforce channel locking on webhooks and enable webhook debugging, and posts from plugins and bots appear in the compliance record, so automations stay capturable. We found no public information on SCIM provisioning, documented rate limits, event subscriptions with retries, or an app directory.

Slack

A real ecosystem is implied — named integrations, SCIM provisioning, SAML SSO — and I welcome the workspace-level power to allow or restrict third-party services. But the evidence documents no REST API, no webhooks, no bot framework and no rate limits, so whether this is buildable without a partner agreement is unproven from evidence.

Mattermost

The contracting entity is Mattermost, Inc., a US company relying on the Data Privacy Framework and Standard Contractual Clauses, with transfers to the United States named, a US-based premier support tier on offer, and Google Analytics on the vendor's sites; we found no public information on EU data residency, an EU contracting entity, or a published subprocessor list for the vendor-managed cloud. The self-hosted and air-gapped options put the archive itself in the customer's hands, which is the only reason this clears the floor. Hosted push notifications and self-hosted telemetry are opt-out, so the European buyer must configure rather than assume.

Slack

The contracting entity is Salesforce, Inc. of San Francisco and no subprocessor chain is named, while the AI stack runs in Slack's own AWS private cloud with no region stated and a US LLM provider sits in the integration list. Germany data residency exists but is purchasable only from Business+ upward — partial residency on top of a non-EU entity is precisely the anchor-3 pattern.

Mattermost

Every plan routes to Contact Sales, Get Pricing or Request Quote, and we found no public per-user price, billing period, VAT treatment or minimum term; only a free limited-use evaluation edition is public. The compliance monitoring and export features a regulated buyer needs sit on the Enterprise and Enterprise Advanced tiers, both unpriced. A buyer cannot compute an annual invoice for any headcount from these pages.

Slack

The pricing page does place compliance features per plan — DLP across plans, Germany residency from Business+, audit logs and legal hold listed, VAT-compliant invoices downloadable. But Enterprise Key Management is an unpriced add-on, the evidence shows no per-user figures or minimum term, and I cannot tell which tier legal hold actually sits in, so the annual invoice for my compliance setup is not computable from what is captured.