We're are planning to deploy Zimbra soon but before that we want to implement a postfix mail gateway that will send messages to users on our current server and new server. The users would all have @example.com

I'm unsure how I would have the gateway determine how user1@example belongs to which server. I have attempted to use transport_maps but not sure how I would do it on a per user basis instead of per domain. I know this is not a postfix forum. I'd just like some help being pointed in the right direction to do this. Any help would be appreciated.