The simplest and most common type of e-mail configuration you can build is a single DNS domain with an Internet connection and a system that acts as a combined mail hub and a gateway. To support this simple configuration, you need only two different sendmail configuration files.
One configuration file is used on a single server that acts as both a mail hub and a gateway to the Internet.
The second configuration file is used on all other hosts. These hosts are dumb clients that defer all decisions to the domain's mail hub and gateway.
For this example, the domain name is starlight.com, and the system that acts as the mail hub and gateway is named mailhost.starlight.com. All outgoing e-mail is rewritten ...