Users have a love-hate relationship with email: they love to use it, and hate when it doesn't work. It's the system administrator's job to make sure it does work. That is the job we tackle in this chapter.
sendmail is not the only mail transport program; smail and qmail are also popular, but plain sendmail is the most widely used mail transport program. This entire chapter is devoted to sendmail, and an entire book can easily be devoted to the subject. In part, this is because of email's importance, but it is also because sendmail has a complex configuration.
 See sendmail by Costales and Allman (O'Reilly & Associates) and Linux Sendmail Administration by Craig Hunt (Sybex) for book-length treatments of sendmail.
Oddly enough, the complexity of sendmail springs in part from an attempt to reduce complexity by placing all email support in one program. At one time, a wide variety of programs and protocols were used for email. Multiple programs complicate configuration and support. Even today, a few distinct delivery schemes remain. SMTP sends email over TCP/IP networks; another program sends mail between users on the same system; still another sends mail between systems on UUCP networks. Each of these mail systemsSMTP, UUCP, and local mailhas its own delivery program and mail addressing scheme. All of this can cause confusion for mail users and for system administrators.