Server wide we have a rule for removing items over 1 day old for the deleted items folder and over 30 days old for the junk mail folder while allowing domains to override auto-clean settings.
Domain level it inherits the system policy and allows users to override auto-clean settings.
I noticed that at both the user and domain level I cannot seem to add a new rule for the Inbox folder, it only lists Deleted Items, Junk Email and Sent Items. A client wants to clean out the oldest content when their inbox is full which is why we stumbled across this trying to add it at the user level.
Currently on SmarterMail Enterprise Build 7619.
Not sure if it requires a parent rule in place for a particular folder before it can be overridden at the user or domain level (we definitely don't want to put a rule server or domain wide for the inbox folder though!).