the variables that control moving messages tagged as spam are zimbraSpamHeader and zimbraSpamHeaderValue. These are, unfortunately global config, so changes to them will affect the whole system. By default we set these to "X-Spam-Flag" and "yes". If you unset them, then messages should go to inbox. But, like I said, this is global to all users. Users can still write a user filter to move spam to junk based on the subject headers etc. It is a tradeoff between POP3 users seeing their spam vs other users not seeing spam by default (or having to create a custom rule).