Users who participate in conversations that are part of the chat list in Teams must have an Exchange Online (cloud-based) mailbox for an admin to search chat conversations. That's because conversations that are part of the chat list are stored in the cloud-based mailboxes of the chat participants. If a chat participant doesn't have an Exchange Online mailbox, the admin can't search or place a hold on chat conversations. For example, in an Exchange hybrid deployment, users with on-premises mailboxes might be able to participate in conversations that are part of the chat list in Teams. However, in this case, content from these conversations isn't searchable and can't be placed on hold because the users don't have cloud-based mailboxes. (For more, see How Exchange and Microsoft Teams interact.)