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).