Sendmail: Theory and Practice, Second Edition
A comprehensive guide to the nooks and crannies of Sendmail, the most-popular electronic mail utility for those using Unix-based servers.
TABLE OF CONTENTS Sendmail: Theory and Practice, Second Edition
Chapter 1: Background and History
While many people use electronic mail (hereafter "e-mail") in today's internetworked world, comparatively few people are concerned with the details of its delivery. This is as it should be. Just as we don't have to know the details of "physical mail" (or postal mail, hereafter, "p-mail") to be able to use it, we would normally prefer to go about our business of using e-mail without knowing its inner workings. In e-mail, as in p-mail, only the Postmasters have to know how it works. Sendmail is a tool of the e-mail Postmaster, and it has handled more e-mail than any comparable tool. Sendmail is to e-mail as delivery vans, airplanes, sorting machines, letter carriers, and all the rest of the postal system are to p-mail. Even if you had received only one e-mail message in your life, chances are better than even that Sendmail had a hand in its delivery.
1.1. A Brief Introduction to Sendmail
On the Internet, the majority of servers run the UNIX operating system or some derivative thereof (such as BSD, Linux, SunOS, ULTRIX, HPUX, or OSF/1). Sendmail is the program most of these systems use to handle e-mail. Some systems include alternate mail transport agents such as MMDF (which ships with SCO UNIX) or SMAIL (which ships with Interactive 386/ix), but even on these systems you will very likely find Sendmail lurking in a corner somewhere, deprecated by the vendor but available if you want it.
Sendmail was written by Eric Allman to handle the problems...
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