I thought there was a checkbox in the Synchronization configuration to optionally create an inbox document tray for any new users which are created by user synchronization. Am I wrong? Was it there and now it's not in 6.12? In some environments, the desired behavior is for this to not happen, rather, to let the admin take care of all the document trays.
What's more, these auto-created "Inbox" document trays are invisible to the "Organization Administrator" role which can complicate things.