A mail configuration requires three elements, which you can combine on the same system or provide in separate systems.
A mail host - A system that is configured to handle email addresses that are difficult to resolve
A minimum of one mail server - A system that is configured to hold one or more mailboxes
Mail clients - Systems that access mail from a mail server
If users are to communicate with networks outside your domain, you must also add a fourth element, a mail gateway.
Figure 24-1 shows a typical electronic mail configuration, using the three basic mail elements plus a mail gateway.
Each element is described in detail in "Hardware Components".