The only suggestions I've ever seen on this topic were to set a forward in the user account panel to something like "archive@mydomail.com" and have all incoming email drop a copy in there. This, however, does not address the outbound copying. On qmail forums, I've seen suggestings for recompiling qmail with logging switch turned on. I am not sure how that would impact a sme server, tho. Good luck.
Jean