Migrate from Microsoft Teams to Mattermost

There is no Teams importer to rely on. Recreate the channels that people actually use, and move the files by hand.

Time: A few days for a small team. A tenant with years of chats is a filing project, not a weekend.

Export

Step 1
List the teams that matter
Write down the teams and channels people opened this month. History in abandoned channels is not worth a manual copy.
Step 2
Download the files
Teams files live in SharePoint or OneDrive. Download the libraries you still open. A chat transcript is not a file store.
Step 3
Save meeting recordings you need
Recordings do not move into Mattermost Calls. Download the ones someone will ask for, and put them with the files.

Import

Step 1
Install Mattermost Team Edition
Create the team and the channels before anyone logs in. Use a staging server if this is the company chat, not a trial.
Step 2
Invite by email
Accounts are new. Tell people the password is not their Microsoft password, and turn off the Teams notifications the same day you ask them to switch.
Step 3
Post the file links
Put the moved files somewhere Mattermost can reach, or attach the few that belong in a channel. Do not promise the old Files tab.

Chat history
Mattermost documents a Slack import. It does not document a Microsoft Teams import. Old messages stay in Teams unless you copy the ones that matter into a channel post.
Meetings
Calls are not a Teams meeting. Calendar invites, breakout rooms, and webinar settings do not transfer. Book the replacement call in whatever calendar you still use.
Apps and tabs
Website tabs, Planner, and the Teams app catalog do not come across. Rebuild the one or two workflows people notice in the first week.

Treat Teams as read-only once the new channels exist. Otherwise Monday's decision lands in whichever app that person still had open.