Sendmail is a very popular mail
transfer agent (MTA) used on the
Internet, handling a very large percentage of all Internet-routed email
at some point as it moves from one host to the other. Other
mail transfer agents do exist (and can be used well with Red Hat Linux), but
most administrators elect to use Sendmail as their MTA
due to its power, scalability and compliance to Internet standards.
Sendmail's core duty, like other MTAs, is to
safely move email between hosts, usually utilizing the Simple Mail
Transfer Protocol (SMTP). However,
Sendmail is highly configurable, allowing you
to control almost every aspect of how email is handled.
Sendmail's roots can be traced to the birth
of email, occurring in the decade before the birth of ARPANET, the
precursor to the Internet. In those days, every user's mailbox was a
file that only they had rights to read, and mail applications simply
added text to that file. Every user had to wade through their mail file to find any
old mail, and reading new mail was a chore. The first actual transfer 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.
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, today's
users actually require much more than simple email delivery. They almost
always desire to interact with their email using a mail user
agent (MUA) that utilizes the
Post Office Protocol
(POP), Internet Message Access Protocol
(IMAP), or even the Web. These
other protocols 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 chapter to go into all that
Sendmail should or could be configured to
do. Rather, consult the many excellent online and offline
sources of information on Sendmail in order
to shape it to fit your exact specifications. You should, however,
understand what files are installed with
Sendmail by default on your system, know how
to make basic configuration changes, be aware of how to stop unwanted email
(spam) being sent through Sendmail, and know
how to extend Sendmail with the
Lightweight Directory Access Protocol
(LDAP).
