Need help with 2 domains and 2 exchange servers sending email

I have 2 seperate domains, and one is a child of the other, ie abc.com and xyz.abc.com. They both exist on a on the same private network. The abc.com domain has a Domain Controller that is also the DNS server, and a Exchange Server which is a member server. The xyz.abc.com is set up the same way. Both domains have 2 xp client hosts on them. jane doe and john doe both a member of the abc.com domain can email each other. Jane Smith and John smith on xyz.abc.com can also email each other. I would like for the users on abc.com to be able to email the users on zyz.abc.com. How do I set this up is there a configuration to set up on each exchange box or do I need to set up something on the DNS box’s. Are MX records the answer?