Teams Private Channels: Group-Based Compliance Model & Purview eDiscovery Considerations

Microsoft Teams Private Channels are undergoing an architectural change that will affect how your organisations uses Microsoft Purview eDiscovery to hold and discovery these messages going forward. In essence, copies of private channel messages will now be stored in the M365 Group mailbox, aligning their storage with how standard and shared channels work today.

This shift, due to roll out from early October 2025 to December 2025, brings new benefits (like greatly expanded channel limits and meeting support) and has the potential to impact your Purview eDiscovery searches and legal holds workflows.

In this blog post, we’ll break down what’s changing, what remains the same, and provide you with the information you need to review your own eDiscovery processes when working with private channel messages.

What’s Changing?

Private channel conversation history is moving to a group-based model. Historically, when users posted in a private channel, copies of those messages were stored in each member of the private channel’s Exchange Online mailbox (in a hidden folder). This meant that Microsoft Purview eDiscovery search and hold actions for private channel content had to be scoped to the member’s mailbox, which added complexity.

Under the new model rolling out in late 2025, each private channel will get its own dedicated channel mailbox linked to the parent Teams’ M365 group mailbox. In other words, private channel messages will be stored similarly to shared channel messages; where the parent Teams’ M365 group mailbox is targeted in eDiscovery searches and holds, instead of targeting the mailboxes of all members of the private channel. Targeting the parent Teams’ M365 Group mailbox in a search or a hold will extend to all dedicated channel mailboxes for shared and private channels within the team as well as including any standard channels. After the transition, any new messages in a private channel will see the message copy being stored in the channel’s group mailbox, not in users’ mailboxes.

Why the change? This aligns the retention and collection of private channel messages to standard and shared channel messages. Instead of having to include separate data sources depending on the type of Teams channel, eDiscovery practitioners can simply target the Team’s M365 Group mailbox and cover all its channel, no matter it’s type.

This update will introduce major improvements to private channels themselves. This includes raising the limits on private channels and members, and enabling features that were previously missing:

• Maximum private channels per team: increasing from 30 to 1000.
• Maximum members in a private channel: increasing from 250 to 5000.
• Meeting scheduling in private channels: previously not supported, now allowed under the new model.

The table below summarizes the old vs new model for Teams private channel messages:

Things to know about the changes

During the migration of Teams private channel messages to the new group-based model, the process will transfer the latest version of each message from the private channel member’s mailbox to the private channel’s dedicated channel mailbox.

However, it’s important to note that this process does not include the migration of held message versions; specifically, any messages that were edited or deleted prior to the migration. These held messages, due to a legal hold or retention policy, will remain in the individual user mailboxes where they were originally stored.

As such, eDiscovery practitioners should consider, based on their need, including the user mailboxes in their search and hold scopes.

Legal Holds for Private Channel Content

Before the migration, if you needed to preserve a private channel’s messages, you placed a hold on the mailboxes of each member of the private channel. This ensured each user’s copy of the channel messages was held by the hold. Often, eDiscovery practitioners would also place a hold on the M365 group mailbox to also hold the messages from standard and shared channels

After the migration, this workflow changes: you will instead place a hold on the parent Team’s M365 group mailbox that corresponds to the private channel.

• Before migration: It is recommended to update any existing hold that are intended to preserve private channel messages so that it includes the parent Team’s M365 group mailbox in addition to the private channel members’ mailboxes. This ensures continuity as any new messages (once the channel migrates) will be stored in the group mailbox.
• After migration: For any new eDiscovery hold involving a private channel, simply add the parent Teams’ M365 group mailbox to the hold. As previously discussed eDiscovery practitioners should consider, based on need, if the hold also needs to include the private channel members mailboxes due to non-migrated content.

Any private channel messages currently held in the user mailbox will continue to be preserved by the existing hold, but to hold any future messages sent post migration will require a hold placed on the group mailbox.

eDiscovery Search and Collection

Performing searches related to private channel messages will change after the migration:

• Before Migration: To collect private channel messages, you targeted the private channel member’s mailbox as a data source in the search.
• After migration: The private channel messages will be stored in a channel mailbox associated with the parent Team’s M365 group mailbox. That means you include the Team’s M365 group mailbox as a data source in your search. As previously discussed eDiscovery practitioners should consider, based on need, if the search also needs to include the private channel members mailboxes due to non-migrated content.

What Isn’t Changing?

It’s important to emphasize that only Teams private channel messages are changing in this rollout. Other content locations in Teams remain as they were, so your existing eDiscovery processes remain unchanged:

• Standard channel messages: These are been stored in the Teams M365 group mailbox. You will continue to place holds on the Team’s M365 group mailbox for standard channel content and target it in searches to do collections.
• Shared channel messages: Shared channels messages are stored in a channel mailbox linked to the M365 group mailbox for the Team. You continue to place holds and undertake searches by targeting the M365 group mailbox for the Team that contains the shared channel.
• Teams chats (1:1 or group chats): Teams chats are stored in each user’s Exchange Online mailbox. For eDiscovery, you will continue to search individual user mailboxes for chats and place holds on user mailboxes to preserve chat content.
• Files and SharePoint data: Any file shared in teams message or uploaded to a SharePoint site associated with a channel remains as it is today.

In conclusion

For more information regarding timelines, refer to the to the Microsoft Teams blog post “New enhancements in Private Channels in Microsoft Teams unlock their full potential” as well as checking for updates via the Message Center Post MC1134737.