I work in e-discovery technology related to all sorts of email formats, and I think there might be a lot left to be discovered.
With Exchange, when one hard-deletes (shift-delete or delete from "Deleted Items" folder) an email (plain delete is a soft-delete, which only moves the email to "Deleted Items" folder), the email is not __really__ deleted. It's put in the system dumpter/tombstone. Now the default retention policy of such messages on Exchange server is 7 days, so if they have weekly backup, it's pretty easy to dig out those deleted emails.
I guess now they should hire a e-discovery firm and if they have any email backup tapes, I am sure a lot more emails can be discovered.