Questions regarding registering a domain

I have a Windows 2003 AD sever in which the domain was named domainname.local. I would like to add exchange server to this to get email from the outside world. I am confused as to how this might happen for a couple of reason, 1) the domainname.local’s corresponding domainname.com is already registered to someone else, so I will have to register a domain name that doesn’t match what is in AD and 2)How will exchange get emails from the outside world if it is part of domainname.local as opposed to a .com domain? Any thoughts/ideas would be great.

Thanks
Paul