How do I set up Exchange to receive emails once I change the MX records

We just got a static IP and are changing the MX records on our external webhost to point to our internal Exchange server 2003 on SBS 2003 that all emails can be received via exchange as opposed to just internal office emails. I looked around and it just seems like I have to make sure that there are accounts on the exchange server that reflect the domain correct? So an email going to person1@domain.com should have an account on the exchange server that is person1@domain.com? Is that all I have to do, or is there more to it?