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 they cannot deliver. 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 or in the hosts file in the name service. You must also use the main.cf file as the mail configuration file on the mail host system. For detailed task information, refer to How to Set Up a Mail Host in Chapter 25, Mail Services (Tasks).
A good candidate for a mail host is a system on the local area network that also has a modem for setting up PPP or UUCP links over telephone lines. Another good candidate is a system that is configured as a router from your network to the Internet global network. For more information, refer to Chapter 29, Solaris PPP 4.0 (Overview), Chapter 38, Overview of UUCP, and “Configuring Routers” in System Administration Guide: IP Services. If none of the systems on your local network has a modem, designate one as the mail host.
Some sites use standalone machines that are not networked in a time-sharing configuration. That is, 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 one-system network. Overview of the Hardware Components in Chapter 24, Mail Services (Overview) provides a figure that shows a typical email configuration.