Email addres display problem

I setup to exchange 2000 one in root domain (xx.com) and second at child domain (yy.xx.com).
Both of the email server can send and received email. But how i can I change the user email address display on child domain to be user@yy.xx.com currently both of user carry user@xx.com display email address. I did try to change at recepient policy but it will change all the user including root domain. BTW both exchange server in same OU.