Internal and External Domain – need your help…..

We have a server setup here which is for our internal domain and internal DNS. This server runs on windows 2003 and we use AD. Now we have a new server for the purpose of publicly hosting our website, email and DNS. I want to use exhange 2003 for the email. Since exhange relies on AD also, I just wanted to ask if do I need to setup our internal domain in that new server? If the internal and external domain will be on different server, how can the users connect thru their email?