This article is contributed. See the original author and article here.

Disclaimer

This document is not meant to replace any official documentation, including those found at docs.microsoft.com. Those documents are continually updated and maintained by Microsoft Corporation. If there is a discrepancy between this document and what you find in the Compliance User Interface (UI) or inside of a reference in docs.microsoft.com, you should always defer to that official documentation and contact your Microsoft Account team as needed. Links to the docs.microsoft.com data will be referenced both in the document steps as well as in the appendix.

All the following steps should be done with test data, and where possible, testing should be performed in a test environment. Testing should never be performed against production data.

Target Audience

Microsoft customers who want to better understand Microsoft Purview.

Document Scope

The purpose of this document (and series) is to provide insights into various user cases, announcements, customer driven questions, etc.

Topics for this blog entry

Here are the topics covered in this issue of the blog:

Applying Retention Policies to a Teams Channels

Out-of-Scope

This blog series and entry is only meant to provide information, but for your specific use cases or needs, it is recommended that you contact your Microsoft Account Team to find other possible solutions to your needs.

Applying a Retention Label Policy Teams Channels

Overview

By default, you can set up Retention Policies for Teams Channels which is applied at the Team level for ALL channels under a single team, NOT a single channel under a team.

The Note below is from the following Microsoft documentation:

Information Point #1

Learn about retention for Teams – Microsoft Purview (compliance) | Microsoft Learn

This Microsoft Link explains how the storage on the backend works for Teams Chats.

Learn about retention for Teams – Microsoft Purview (compliance) | Microsoft Learn

Below are some excerpts that I find to be of value in understanding out this retention operates.

Information Point #2

“You can use a retention policy to retain data from chats and channel messages in Teams, and delete these chats and messages. Behind the scenes, Exchange mailboxes are used to store data copied from these messages. Data from Teams chats is stored in a hidden folder in the mailbox of each user included in the chat, and a similar hidden folder in a group mailbox is used for Teams channel messages. These hidden folders aren’t designed to be directly accessible to users or administrators, but instead, store data that compliance administrators can search with eDiscovery tools.

These mailboxes are, listed by their RecipientTypeDetails attribute:

UserMailbox: These mailboxes store message data for Teams private channels and cloud-based Teams users.

MailUser: These mailboxes store message data for on-premises Teams users.

GroupMailbox: These mailboxes store message data for Teams standard channels.

SubstrateGroup: These mailboxes store message data for Teams shared channels.”

Information Point #3

“Although this data from Teams chats and channel messages are stored in mailboxes, you must configure a retention policy for the Teams channel messages and Teams chats locations. Teams chats and channel messages aren’t included in retention policies that are configured for Exchange user or group mailboxes. Similarly, retention policies for Teams don’t affect other email items stored in mailboxes.”

Information Point #4

“After a retention policy is configured for chat and channel messages, a timer job from the Exchange service periodically evaluates items in the hidden mailbox folder where these Teams messages are stored. The timer job typically takes 1-7 days to run. When these items have expired their retention period, they are moved to the SubstrateHolds folder—another hidden folder that’s in every user or group mailbox to store “soft-deleted” items before they’re permanently deleted.

Messages remain in the SubstrateHolds folder for at least 1 day, and then if they’re eligible for deletion, the timer job permanently deletes them the next time it runs.”

Information Point #5

Overview of security and compliance – Microsoft Teams | Microsoft Learn

Questions and Answers

Question #1 – What if I have an existing Team (or Teams) and for each Channel under that Team, I want to apply a DIFFERENT retention Ppolicy? Or in other words, I do not want to reconfigure my Team(s) to have 1 Channel mapped to 1 Team and therefore be able to map 1 Retention policy to that Channel.

Answer #1 – At the writing of this blog entry, because of the underlying architecture of how Teams Channel message are stored (See Information Points #1 and #2 above) there is currently NO method to apply a Retention Policy to an individual Channel under a Team.

Question #2 – Follow-up, I cannot even do this with Adaptive Scopes?

Answer #2 – The answer is still currently NO. Adaptive scopes do not have attributes that apply to Teams Channels specifically. Here is a summary of attributes and properties used in Adaptive scopes.

Question #3 – Do I have any other way to delete data from Teams Channels?

Answer #3 – Yes and No. Through the Adaptive Scopes mentioned above, you can apply retention policies to users’ mailboxes and thus the data held within those mailboxes. However, this approach would limit those retention policies to the users specified AND to all their email data, not just one specific Teams Channel data held in the Substrate. Refer to Information Point #2 above to see how Teams Channel date is organized and stored in M365 tenants.