Molok,
I don't think we'd really have any control for outbound emails from users because they could do it from anywhere. But for inbound messages, on the mail server, you can create a .forward file under the user's home dir and enter the following in the file.
username, root
It should automatically forward a copy of whatever the user sends out to the root mailbox as long as the user and the root are on the same box, though you could replace the user root with root@domain.com it it's on another box.
Hope this help, anyone is welcome to correct me if I made a mistake.
Joe