A mail host is the machine that you designate as the main mail machine on your network. A mail host is the machine to which other systems at the site forward mail that cannot be delivered. You designate a system as a mail host in the hosts database by adding the word mailhost to the right of the IP address in the local /etc/hosts file. Alternately, you can add the word mailhost similarly to the hosts file in the name service. For detailed task information, refer to How to Set Up a Mail Host in Chapter 13, Mail Services (Tasks).
A good candidate for a mail host is a system that is configured as a router from your network to the Internet global network. For more information, refer to Chapter 15, Solaris PPP 4.0 (Overview), Chapter 24, UUCP (Overview), and Configuring an IPv4 Router in System Administration Guide: IP Services. If no systems on your local network have a modem, designate a system as the mail host.
Some sites use standalone machines that are not networked in a time-sharing configuration. Specifically, the standalone machine serves terminals that are attached to its serial ports. You can set up electronic mail for this configuration by designating the standalone system as the mail host of a single-system network. Overview of the Hardware Components in Chapter 12, Mail Services (Overview) provides a figure that shows a typical email configuration.