Start Exchange Administrator and go to Mailbox Resources under your private information store (server level) in the Exchange Admin program. Make the right hand pane active and then do a File | Save Window Contents. This will allow you to dump info on user’s mailboxes including last logon/logoff time. There are a number of useful fields in the default view or you can add columns/fields doing a view|columns also in Exchange Admin.
Once in CSV format you can massage/sort data using Excel. Select mailboxes to be deleted based on the amount of time inactive. You can purge old mail held in mailboxes using the “clean mailbox function” under the tools menu in the admin program.