I'm trying to migrate my mail server from kerio connect to ZCS. Since I'm beginner in zimbra, I want to perform migration with the simultaneous existence of these two servers, zimbra as primary and kerio as backup mail server in order of their DNS MX records priority. So, I manually synced all of my user accounts using imapsync and now I need to keep these servers synchronized for some test period (one month). How can I perform this migration plan?