The sendmail program provides the following features:
It supports UNIX System V mail, UNIX version 7 mail, and Internet mail.
It is reliable. It is designed to correctly deliver every message. No message should ever be completely lost.
It uses existing software for delivery whenever possible.
It can be configured to handle complex environments, including multiple connections to a single network type (like with UUCP or Ethernet). sendmail checks the contents of a name as well as its syntax to determine which mailer to use.
It uses configuration files to control mail configuration.
Groups can maintain their own mailing lists. Individuals can specify their own forwarding without modifying the domain-wide alias file (typically located in the domain-wide aliases maintained by NIS or NIS+).
Each user can specify a custom mailer to process incoming mail, which can provide functions like returning an "I am on vacation" message. See the vacation(1) man page for more information.
Figure 3-1 shows how sendmail interacts with the other programs in the mail system.
The user interacts with a mail-generating and -sending program. When the mail is submitted, the mail-generating program calls sendmail, which routes the message to the correct mailers. Because some of the senders may be network servers and some of the mailers may be network clients, sendmail may be used as an Internet mail gateway.