If they did, the mailbox user would (in general) be unable to reach them, since they'd be under the admin login. As it is the admin cannot (in general) reach them, since they are under the milabox login! This needs a rethink.
By the way, be warned that there are many problems with the Junk Filter. Most are with Quarantine so I'd suggest you avoid that. But beware some affect the SA options, e.g. it seems the first Policy on the list just doesn't work.