Subdomain Windows

My company has a partner organization that resells our service. They have requested that it appears to their customers that they are contacting them and not us. I need to setup a sub-domain I think. They are mostly concerned about email and are happy to have something like partner.mydomain.com for email addresses. To me, that doesn’t really do much to disguise what company people are contacting, but it is what they asked for. I have an AD 2003 and Exchange 2003 setup for my primary domain and I want to avoid any hardware or licensing costs whenever possible. What are my options here? Any suggestions are appreciated as I have no experience setting up a subdomain.