Exchange Server 2010 has the option to search for specific content in mailboxes in your Exchange environment. But since this imposes a security risk, and given the fact there's a potential privacy issue this functionality is disabled by default. So, you have to enable it deliberately.
Note. Before enabling the multi mailbox search functionality in Exchange Server 2010 make sure that you're covered by the legal and HR department in your organization to make sure you're covered in case something goes terribly wrong!
To enable the search functionality for an employee in the legal department, for example a user called "Joe S. Lawyer" logon as an Exchange administrator to the Exchange Server 2010 Control Panel (ECP) and select the "manage my organization" option. Select the "Administrator Roles" tab and then the "Discovery Managment" option. Add the "Joe S. Lawyer" user to this Administrator Role.
Logon to OWA using the "Joe S. Lawyer" option and select the Options Page. Joe now also can select the "Manage My Organization" option. In the navigation pane select Mail Control and Joe should see the E-discovery option. Joe is now able to create a new Mailbox Search. The search needs to include the actual Mailbox that needs to be searched, the search needs to have a distinct name and the search should be stored somewhere. By default the "Discovery Search Mailbox" is used for storing the result set.
Please be aware that the searches should be defined very well to prevent a massive result set containing tons of data in the Discovery Search Mailbox. Also, for the Discovery functionality in the Exchange Control Panel (ECP) you need an Enterprise Client Access License (eCAL).
The last step is to grant 'Full Access' permission on the Discovery Search Mailbox to Joe S. Lawyer so he can open this mailbox and view the search results.