Slack, Microsoft Teams, Google Chat and Discord rent the room. Mattermost Team Edition is published under MIT as a single binary with PostgreSQL, and the vendor's own edition notes describe it for use without single sign-on. Element is AGPL-3.0 and speaks Matrix, which is the path when sign-on matters. Microsoft Teams is also the company's files, meetings and identity. Replacing the chat window does not replace that bundle.
What is the SSO limit?
If the company requires SSO, Mattermost Team Edition is the wrong edition to promise. We do not quietly step them onto a commercial edition and call it open source. Element, or a paid Mattermost edition they knowingly buy, is the honest fork.
What does owned chat not include?
Slack's app directory, and Teams' coupling to the rest of Microsoft 365. A company that lives in those apps will feel the gap on day one. Name it in the scope, or do not migrate.
When should chat stay rented?
When the company is already inside Microsoft 365 and chat is not the system of record. Own chat when the history is sensitive enough that it cannot sit in a vendor's retention policy.
What do we hand over?
We select the system, deploy it on infrastructure you control, connect it to the tools you keep, and write the runbook so your own team can operate it. Support after handover is optional. The rest of the map is in software you can stop renting; the engagement behind it is open-source business systems.
Name one process that still needs a person. We will tell you what a one-time build takes.
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