Exchange Server SMTP Help

Okay, Here is the problem.
Building a Exchange server 2000, and need help with the SMTP. lets say Server name is NTLAW and the The Domain is called DOMAIN. THE ISp has a hold and storage on the mail that sends the mail to the ISDN IP of 63.0.0.0 <-example. HThe users emails are admin@anyname.com, But the server name and domain has to stay the same as above. How do I tell the exchange server to look at the ISP and receive its mail. and then send it to the users mail that is setup in the postoffice. so lets say I set up keithmaddox@keith.com and the username is Keithm on the server. .....HELP