Exchange 2000 DNS question

We are currently adding an Exchange server and I was wondering about DNS configuration. I would like to set it up so the internal clients register with our local DNS server on the domain company.local and our ISP host our external domain of company.com. I assume that the window domain should be the .local one but I was wondering how to configure exchange so that the SMTP addresses use the .com address. Do I just change the recipient policy or is there more involved then that. Thanks for the help.