Mattermost vs Slack

Mattermost

Channels and threads are real enough to have their own API endpoints and even a beta operation to move a thread to another channel, with guest accounts, file previews, and, on the Enterprise plan, shared channels and Matrix-protocol federation; edits and deletions are retained in the record by default, and search is documented at a three-million-post scale on Professional and beyond that on Enterprise. We found no public information on pinned or saved items, search filters by channel, person or date, or channel archiving and merging.

Slack

Channels, Slack Connect with 4M external users, file sharing and AI search across stored company knowledge are evidenced, with documented history limits (free: 1 year, paid: unlimited). But the evidence says nothing about threads, message edit history, or exporting a conversation a human can read — for a chat log to be a record, I need that archive story, not just search.

Mattermost

Transport and at-rest mechanisms are documented concretely — TLS with AES-256 and 2048-bit RSA, disk-level encryption on infrastructure the customer runs, S3 with S3-managed keys, and cluster traffic encrypted with AES-256 — and the deployment guide says plainly that the database is not encrypted at the application layer so that search and compliance reporting keep working, which is the kind of honesty I want more of. Granular role-based access, AD/LDAP sync, SSO, guest accounts and zero-trust channel access are evidenced; we found no public information on end-to-end encryption of message content, device verification, or sessions an administrator can revoke.

Slack

Encryption at rest and in transit with vendor-held keys by default; Enterprise Key Management is only a paid add-on, which is the right direction but not first-class. SSO, 2FA, information barriers and admin control over DM visibility and external connections are documented, but there is no end-to-end mode and no plain statement of what Slack itself can read.

Mattermost

Compliance export runs in formats a lawyer already uses — Global Relay EML, Actiance XML, Proofpoint — tracking each message by ID through edits and deletions, with job status visible and every query and download action logged in an audit history explicitly to prevent unauthorised queries; that last part is the co-determination instinct done right. Data Retention Policy and Legal Hold are named at the Enterprise tier and self-hosted telemetry can be opted out; but the default retains everything including deletions rather than deleting, and we found no public information on presence a person can control or on activity analytics and whether they can be switched off organisation-wide.

Slack

Retention is configurable at workspace and channel level, and legal hold, audit logs and native DLP exist — but the evidence is silent on discovery export in any format, on whether deletion is evidenced, on user-controlled presence, and on switching analytics off organisation-wide. A works agreement cannot rest on half of rubric level 5; those absences are information.

Mattermost

Custody really can be the customer's: the same server installs via Kubernetes, Linux or containers, an air-gapped runbook with a bill of materials and registry mirroring is published, FIPS-compliant STIG-hardened images exist, and the customer stands up the database, file storage and TLS themselves — with Matrix-protocol federation and Slack-compatible webhooks giving paths in and past message history exportable via a command-line tool. The compliance export excludes Playbooks and Boards data, and we found no public information on a complete single-format export of an entire workspace.

Slack

Cloud-only and proprietary: no self-hosting, no open source, no federation, and no documented full export anywhere in the evidence — not even the partial one rubric level 0 would still allow scoring. Germany data location from Business+ and LLMs in Slack's own AWS cloud are residency, not custody; the archive belongs to Salesforce either way.

Mattermost

A versioned REST API documented endpoint by endpoint with its permission requirements, incoming and outgoing webhooks available on every plan including the entry edition, Slack-compatible webhook payloads, interactive dialogs and Mattermost Blocks, plus admin-enforced channel locking for webhooks — a control I am glad to see. We found no public information on an app directory, SCIM provisioning, documented rate limits, or an event-subscription model with retries.

Slack

SSO, SCIM provisioning, a workflow builder usable with or without code, and named integrations (Google Drive, ChatGPT, Asana, Workday) with workspace-level allow/restrict control are all evidenced. But the evidence captures no documented API, no webhook model, no rate limits or sandbox — I cannot credit a platform on integration logos alone.

Mattermost

The captured pages show a United States vendor relying on the EU–U.S. Data Privacy Framework and Standard Contractual Clauses, personal information transferred to the United States, Google Analytics on the sites, Azure and AWS named as cloud hosting for US-headquartered providers, and no published list of the subprocessors behind the vendor-managed cloud. The documented air-gapped, self-hosted path with telemetry opt-out is what removes the question in practice for a European deployment, but the contracting entity sits outside the EU and we found no public information on EU hosting or an EU entity.

Slack

Germany data residency exists but only from Business+ upward, while the contracting entity is Salesforce, Inc. of San Francisco and the subprocessor chain is unnamed — exactly the rubric level 3 situation. The LLMs run in Slack's own AWS private cloud and ChatGPT is a showcased integration with no legal basis or safeguard stated for that reach.

Mattermost

The captured pricing page routes every named plan to a sales conversation — Professional to "Contact Sales", Enterprise to "Get Pricing", Enterprise Advanced to "Request Quote" — and we found no public per-user price for any tier anywhere in the captures. A limited-use free edition of Enterprise Advanced is offered for technical evaluation, but no buyer can compute an annual invoice from these pages.

Slack

Tier boundaries are genuinely public — which plan gets unlimited history, legal hold, audit logs, SCIM, Germany hosting and which SLA — and VAT-compliant invoices are downloadable. But not a single per-user price figure appears in the captured pricing pages, and Enterprise Key Management is an unpriced add-on, so the annual invoice for our headcount is not computable from what is published here.