MC1134737 as an example of eDiscovery Challenges

The changes to private channels are an improvement for end-users. What will it bring for eDiscovery?

Photo by Pierre Bamin on Unsplash

I am often asked how people are expected to keep up with the changes in M365 as they relate to eDiscovery. For many outside the M365 world, that might seem like an odd thing, but I wanted to take a moment to highlight how this one change leads to a series of questions, and how some of those questions remain unanswered.

MC1134737 is related to a change in Teams Private Channels:

Summary

Microsoft Teams private channels will transition to using a channel mailbox, increasing limits to 1000 channels per team and 5000 members per channel, with meetings supported. Compliance policies will shift from user mailboxes to the team’s group mailbox. Migration starts October 2025; admins must update policies by September 20, 2025.

That was originally published on August 13, 2025.

On September 18th, there was an update in the timeline:

When this will happen:

General Availability (Worldwide): We will begin rolling out early October 2025 (prevously late September) and expect to complete by mid-December 2025.

So, what should eDiscovery practitioners be prepared for?

Purview eDiscovery and Legal Hold

With this change, private channel data will move from user mailboxes to the group mailbox, aligning with how shared channels data is stored. Previously, legal holds were applied to user mailboxes since private channel data resided in each member’s mailbox. Post-migration, the latest version of private channel message data from each user’s mailbox is moved to the private channel’s group mailbox. Message edits and deleted messages will not be copied into the group mailbox.

Note: For users on hold, copies of message edits and deleted messages will remain in the user’s preserved library folder until the hold expires, while the latest version of private channel message data from each user’s mailbox is copied to the private channel’s group mailbox.

Before migration

If a legal hold exists for a private channel, admins must also apply the policy to the team’s group, in addition to the policy already applied to users’ mailboxes, in order for a hold to apply to new data generated and stored in the private channel’s mailbox. Note that policies applied to the team’s group can apply to all channels in the team.

After migration

For existing holds admins will need to ensure that the hold is applied to the user mailbox as well as the new group mailbox, in order for a hold to apply to existing and new message data. Any new legal holds will need to be applied to the group.

For complete eDiscovery, include both the private channel’s users’ mailboxes and the team’s group mailbox to retrieve pre-migration messages, message history, and post-migration new content.

Do you have all of that? Sometime over the next couple of months, how we preserve and collect messages from Teams Private Channels will change. That change may come to your tenant sooner or later than it does to mine. It’s a mystery.

Understanding the changes is crucial if you are required to deal with private channels in your eDiscovery workflow. This change has a significant impact, and yet we can’t say with certainty when you’ll see it.

Additionally, we may need to wait and see if it works precisely as described in the MC. That’s not always been true in the past. Testing to verify will occur once it’s rolled out, so we will all need to be on our toes to ensure we’re testing before we need to preserve and collect from private channels. This is the challenge we live with every day.

Let’s get into the details a bit further, as I expect to see them when I test. This, of course, is subject to change!