Mattermost vs Slack

Mattermost

Channel-based collaboration, real threaded discussions with a documented thread API and per-endpoint permissions, file sharing with link and file previews, and search documented up to 3 million posts — and the default retains every message including edits and deletes, so no history cap bites. I found no public information on pinned or saved items or search filtered by channel, person and date, and moving a thread to another channel is flagged as beta subject to change, so I stop just above the solid baseline.

Slack

Slack Connect delivers genuine cross-organisation channels, unlimited message history on paid plans, and Enterprise/AI search across stored knowledge — real 8-anchor material. But edit history, pinned items, filtered search and channel lifecycle (archive/rename/merge) are nowhere in the evidence, so it tops out just under that anchor.

Mattermost

Transport TLS with named ciphers, at-rest encryption via disk or storage-level methods within your own infrastructure, granular role-based access with AD/LDAP sync, SSO and MFA are all documented, and the pages state plainly that the database itself is unencrypted so search and compliance reporting work — plus exactly what the optional hosted push service sees. I found no public information on end-to-end encryption, device verification, or admin-revocable session management, which is what separates the operator-reads-everything mode from the top anchors.

Slack

Encryption at rest and in transit (vendor keys), SAML SSO down to the free tier, two-factor, information barriers and per-channel posting permissions — that's the 5 anchor. No end-to-end encryption mode, no device verification, no session revocation, and no statement of what Slack itself can read, so it cannot go higher.

Mattermost

Legal hold and eDiscovery automation are named plan features, and the compliance export documentation is the real thing: CSV, Actiance XML, Global Relay EML and Proofpoint formats, edit and deletion tracking by message identifier across export batches, configurable daily jobs with visible job status, and an audit history of every compliance query and download. I found no public information on organisation-wide switching of activity analytics or user-controlled presence, and Playbooks and Boards sit outside the compliance export — the captured pages stop me short of the works-agreement grade.

Slack

Retention configurable at workspace, channel and other levels, audit logs, legal hold and native DLP are all evidenced — most of the 8-anchor list. But the evidence is silent on any export format a lawyer could use, on retention deletion being evidenced, and on whether activity analytics can be switched off, which caps it below 8.

Mattermost

This is a supported deployment, not a hobby build: Kubernetes, Linux and container paths install the same server, high availability is available self-managed, a FIPS-compliant STIG-hardened image of every container is published, and the air-gap runbook goes down to a bill of materials and private registry mirroring. Federation via Matrix protocol interoperability and a command-line past-history export with a from-timestamp option put custody genuinely with the customer; I found no public information on migration-in tooling or deeper federation detail.

Slack

Cloud-only and proprietary: no self-hosted edition, no open-source core, no federation — Slack Connect is a feature of their cloud, not an exit. The closest things to custody are the Enterprise Key Management add-on and a Germany data-location option, neither of which hands the customer the archive, and the evidence documents no full export format at all.

Mattermost

A versioned REST API with bearer authentication and permission requirements documented per endpoint, incoming and outgoing webhooks that are Slack-compatible for migration, interactive blocks and dialogs for structured interaction, and admin-enforced channel locking on webhooks — real permission controls where the pages speak. I found no public information on SCIM provisioning, documented rate limits, event subscriptions with retries, a sandbox, or a plugin framework whose permissions a customer can audit, which is what my upgrade path depends on.

Slack

A real platform: a flexible build-your-own foundation, workflows by click or code, vendor-named integrations, SCIM provisioning, and workspace-level allow/restrict control over third-party services. What keeps it off 10 is the absence of a documented versioned API with deprecation policy, rate limits, and a sandbox in the evidence.

Mattermost

The contracting entity is Mattermost, Inc., the privacy framework is built around US transfer mechanisms with US-based premier support on offer, and I found no public information on EU data residency or a published subprocessor list for the vendor-managed offering. What saves it for me is the self-hosted path, documented as customer-controlled processing, with air-gap operation and telemetry that can be opted out — run it that way and the question mostly disappears, but that is my work, not the vendor's chain.

Slack

Salesforce Inc. of San Francisco is the vendor and contracting entity, subprocessors are unnamed in the evidence, and EU residency is a Germany checkbox that only starts at Business+ — the anchor-3 case exactly. The LLM stack runs in Slack's own AWS private cloud with no EU claim, and there is no self-hosted option that would remove the question.

Mattermost

Every plan ends in a sales conversation — Professional says Contact Sales, Enterprise says Get Pricing, Enterprise Advanced says Request Quote — and I found no public information on any per-user price, so an annual invoice cannot be computed from the pricing page. Credit for a free evaluation edition and unusually clear feature-to-plan boundaries, but the money is entirely opaque.

Slack

The pricing pages do map real feature boundaries — unlimited history from Pro, SSO from free, DLP on all plans, Germany hosting from Business+, SLA response times per tier — and VAT-compliant invoices are documented. But not one per-user price figure appears in the evidence and Enterprise+ remains unpriced, so the annual invoice is not computable from this sheet.