Mail Service Programs
Mail services are provided by a combination of programs that interact, as shown by the simplified diagram in Figure 3.
Figure 3. How Mail Programs Interact
Users send messages with programs—known as mail user agents (MUAs)—such as mailx, mailtool, or dtmail. See the manual pages for information about these programs.
The message is collected by the program that was used to generate it and is passed to the sendmail daemon or mail transport agent (MTA). The sendmail daemon parses (divides into identifiable segments) the addresses in the message, using information from the configuration file /etc/mail/sendmail.cf to determine ...