From the Internet point of view multiple Internet domains mean multiple MX (mail exchanger) records in the company's DNS database. In most implementations of bind, the DNS daemon, the "database" is a set of related structured text files containing IP address and host name information so that machines can find each other by name based on which Internet domain they are in (or find names by looking up addresses). In the database, various record types are defined for machine addresses, mail exchangers, and other things. The MX records are in the domain database file typically named after the Internet domain name (e.g., GSSNET.DOM). In the file are records in the following format:

Handling email for multiple DNS domains usually means pointing MX records for more than one domain name to a single host. This host must be able to forward or deliver mail to users whose SMTP addresses contain the various domains handled by the company's mail hosts. One way to see what mail exchangers are configured in a DNS is to use the nslookup program, a command line utility available on most UNIX systems:

The output should look something like this:

In this example Internet mail for both the gssnet.com and messagingcentral.com domains is sent to mail1.gssnet.com. Causing mail for multiple domains to be routed to a particular host is a straightforward matter. It's only a question of editing the right text files and adding or modifying the right MX record entries.

Firewalls
Of course there are other considerations, for example sending mail through a firewall. Typically, the front-end host will be placed on a DMZ connected to a firewall but not on the outside of the firewall, as shown in Figure A.

FIGURE A

Here's a diagram of a simplified DMZ Configuration for SMTP Relay Host. Click picture for a larger image.

Of course, the diagram in Figure A is simplified, but it gets at the main issue which is securing internal SMTP services to make them inaccessible to any host other than the SMTP relay or front-end host.