Red Hat LINUX 7.2 Reference Manual page 264

264 Chapter 16:Email
of a mail message file from one host to another didn't take place until 1972, where email began to
to be moved by FTP over the NCP network protocol. This easier method of communication quickly
became popular, even to the point where it made up most of ARPANET's traffic in less than a year.
However, a lack of standardization between competing protocols made email much harder to send
from some systems, and this continued until the ARPANET standardized on TCP/IP in 1982. A new
protocol, SMTP, materialized for message transporting. These developments, combined with HOSTS
files being replaced with DNS, allowed full-featured MTAs to materialize. Sendmail, which grew out
of an earlier email delivery system called Delivermail, quickly became the standard as the Internet
began to expand and be widely utilized.
16.3.2 Purpose and Limitations
It is important to be aware of what Sendmail is and what it can do for you as opposed to what it is not. In
these days of monolithic applications that fulfill multiple roles, you might initially think that Sendmail
is the only application you need to run an email server within your organization. Technically, that is
true, as Sendmail can spool mail to your users' directories and accepts new email via the command
line. But, most users actually require much more than simple email delivery. They usually want to
interact with their email using an MUA that uses POP or IMAP to download their messages to their
local machine. Or, they may prefer a Web interface to gain access to their mailbox. These other
applications can work in conjunction with Sendmail and SMTP, but they actually exist for different
reasons and can operate separately from one another.
It is beyond the scope of this section to go into all that Sendmail should or could be configured to
do. With literally hundreds of different options and rule sets, entire volumes are dedicated to helping
explain everything that can be done and how to fix things that go wrong. You should consult the many
excellent online and offline sources of information on Sendmail in order to shape it to fit your exact
specifications.
However, you should understand what files are installed with Sendmail by default on your system and
know how to make basic configuration changes. You should also be aware of how to stop unwanted
email (spam) and how to extend Sendmail with the Lightweight Directory Access Protocol (LDAP).
16.3.3 The Default Sendmail Installation
While you can download the source code for Sendmail and build your own copy, many users prefer
to use the version of Sendmail installed by default with their Red Hat Linux system. You can also use
the Red Hat Linux CD-ROMs to reinstall the sendmail RPM at a later point.
After installation, the sendmail executable is placed in the /usr/sbin directory.
Sendmail's lengthy and detailed configuration file (sendmail.cf) is installed in /etc. You
should not edit the sendmail.cf file directly unless you know exactly what you are doing, due
to the fact it is very lengthy and complex. Instead, to make configuration changes to Sendmail,