More businesses register multiple domain names than ever before because it's cheap and easy to do. Some users have several e-mail addresses, all with different domain names but delivered via the same mail server. If you aren't hosting more than one domain on your Exchange server, you probably will in the near future.
Follow these steps to configure the Internet Mail Service (IMS) to support multiple domains in Exchange 5.5:
In order for mail from the new domains to be delivered to your Exchange server, you'll need to do some DNS work. The DNS server that's authoritative for each domain you host must point the domain's MX record to the same host A record that contains the IP address of your SMTP host. If you don't host your own DNS, contact your ISP about making these changes.