some users 1 server, some users another

Hi Apologies for the newbie type question. We have a requirement for another server in a branch office, and the powers that be want to have the users email local to that machine. So I have 1 domain name and 2 servers. All external email will come into head office, and email for nominated users will be passed to the branch office server. I'm presuming there is a way to do this with OSX / Postifx, but I cannot find how. Any pointers? TIA! BP