The Top Senders and Receivers Reports can be run under Mailbox in MessageStats Reports and filter on Department to get the Firm or if Firm is the Org, you would select the Organization filter.
Once the Top Senders & Receivers are listed, you can then audit the mailboxes and run the Mailbox Auditing Reports to find out who the Top Senders are sending to and who they are receiving from and vice versa with Receivers.
In order to audit mailboxes, you will have to add the mailboxes to Auditing in the Console under Organization Properties, by right clicking the Org Name and selecting Properties. Once the mailboxes have been added another gathering needs to take place in order to populate the database with this info.
