If your organization has multiple Domain Name System (DNS) names for its server, or is hosting mail for several organizations, you can assign more than one DNS name that will point to the same Exchange server.
This is useful as a way of aggregating e-mail for several organizations through one computer, or for web hosts that want to handle e-mail for multiple domains using a single installation of Exchange.
To do this, first the DNS entries for the organization must all resolve to the same server. This is generally easy enough to do, but it must be done on the DNS server that handles external, not internal, lookups. In other words, this must be done on the DNS server where one's domain information is stored for the outside world, not the internal DNS server used for local /Active Directory lookups. (This may seem obvious, but it is a source of a good deal of confusion among even relatively experienced Exchange administrators.)
Step two: you need to change how Exchange deals internally with the domain names available. Because of this, there are several possible approaches, each with their own benefits and drawbacks. To begin setting this up, go to the Routing tab of the Internet Mail Service (note that this only works in Exchange 5.0 SP2 or higher) and select a primary DNS domain for the site. This should be the one domain name that you want most associated with the site. Set this domain to <inbound >.
The address permutations for the other domains can be set one of two ways. If you also set them to <inbound >, the Internet Mail Service will immediately deliver the message to the Exchange server without changing it in any way, but there must be a matching SMTP proxy address for that domain name on the mailbox. For instance, if you have the domain name utena.com set to <inbound >and mail arrives for firstname.lastname@example.org, the user mailbox must have the proxy address email@example.com set for it in its E-Mail Addresses tab.
The other way to set a domain name's addressing is to set it to direct to the main DNS name associated with that server. For instance, if your Exchange server is set up as utena.com, and you have brillig.com as another DNS entry for that server, you can set the IMS Routing information for brillig.com to utena.com instead of <inbound >. If you choose this method of rerouting inbound mail, the message will be remained with a RCPT TO: value added to it. This involves a separate DNS lookup, and the message has to be moved around internally on the server. However, this doesn't require the presence of multiple SMTP proxy addresses for each destination mailbox. This approach is useful if you aren't worried about RCPT TO: headers and simply want to redirect mail as quickly as possible.
Serdar Yegulalp is the editor of the Windows 2000 Power Users Newsletter.