eDiscovery of chat messages when Exchange is on-prem

Hi,

You need a Exchange hybrid to get this to work. Teams will use a cloud-based mailbox for the on-premises hosted user, so you don't have to move user mailboxes to Exchange Online to get this to work.

Read more about it here

https://docs.microsoft.com/en-gb/microsoft-365/compliance/search-cloud-based-mailboxes-for-on-premises-users?view=o365-worldwide

To store Teams chat data for an on-premises user in a cloud-based mailbox, the user must be assigned a Microsoft Teams license and an Exchange Online Plan license in Office 365 (or Microsoft 365).