Exchange 2010 - Need to gather emails for a court subpoena

I would take a look at: Understanding Multi-Mailbox Search

You're wanting to perform a Discovery Search, also known as a Multi-Mailbox search. I'm going to assume use of the web-based ECP interface, but similar functionality is available from the shell.

First, the user account that needs to perform this needs to be in the Discovery Management role group. This group is by default empty, meaning even Exchange Administrators have no access to perform Discovery Searches when Exchange 2010 is installed. These roles can be modified in the Roles and Auditing portion of ECP. After you add a user to this group, you'll need to log out then log back in for it to take effect.

After you are in the appropriate group, a new option will appear in ECP within the Mail Control section called Discovery. The interface to perform this task is quite intuitive, save one part; you will need to specify a mailbox to copy the search results into. This is called a Discovery Mailbox, which is simply a locked-down mailbox with high quotas that is by default only accessible by members of the Discovery Management group.