Style Conventions

This book uses the following typographical conventions:

Italic

Indicates the names of files, databases, directories, hostnames, domain names, usernames, email addresses, sendmail feature names , Unix utilities, programs, and it is used to emphasize new terms when they are first introduced.

Constant width

Indicates configuration files, commands and variables , m4 macros and built-in commands, and Unix command-line options. It is used to show the contents of files and the output from commands. Keywords are also in constant width .


Constant width bold

Used in examples to show commands or text that you would type.

Constant width italic

Used in examples and text to show variables for which a context-specific substitution should be made. (The variable filename , for example, would be replaced by some actual filename.)

Examples and Patches

The examples from this book and the author's source code patches for qmail and related packages are freely downloadable from the author's web site at:

http://qmail.gurus.com

Comments and Questions

We have verified the information in this book to the best of our ability, but you may find that features have changed (or even that we have made mistakes!). Please let us know about any errors you find, as well as your suggestions for future editions, by writing to:

O'Reilly Media, Inc.

1005 Gravenstein Highway North

Sebastopol, CA 95472

(800) 998-9938 (in the United States or Canada)

(707) 829-0515 (international or local)

(707) 829-0104 (fax)

We have a web page for this book, where we list errata, examples, or any additional information. You can access this page at:

http://www.oreilly.com/catalog/qmail

To comment or ask technical questions about this book, send email to:

bookquestions@oreilly.com

You can sign up for one or more of our mailing lists at:

http://elists.oreilly.com

For more information about our books, conferences, software, Resource Centers, and the O'Reilly Network, see our web site at:

http://www.oreilly.com

You may also write to the author directly at:

qmail@gurus.com

Acknowledgments

I wish to thank my reviewers, Mark Delany and Russell Nelson, for careful reading of the manuscript and many suggestions to improve it. I particularly thank my editor Simon St.Laurent and the staff at O'Reilly for believing my assurances that this book would in fact be finished, despite mounting evidence to the contrary.

Part I: Introduction to Qmail

The first nine chapters provide an introduction to Internet email and qmail. They describe installing and configuring qmail, including advice on setting up a qmail system as a mail hub, converting an existing system from sendmail, and filtering out viruses and spam from incoming mail:

Chapter 1

Chapter 2

Chapter 3

Chapter 4

Chapter 5

Chapter 6

Chapter 7

Chapter 8

Chapter 9

Chapter 1. Internet Email

Despite being one of the oldest applications on the Internet, email remains the Net's "killer application" for most users. For users' email to be sent and delivered, two kinds of programs have to work together, a mail user agent (MUA) ^[1] that a person uses to send and read mail, and a mail transfer agent (MTA) that moves the mail from server to server. Qmail is a modern MTA for Unix and Unix-like systems.

^[1] Popular MUAs include pine and mutt on Unix systems, and Eudora, Netscape, Outlook, and Outlook Express on PCs and Macs.

Before diving into the details of qmail, it's a good idea to closely examine some of the basics of Internet email that apply to all MUAs and MTAs. Common terms like envelope and mailbox have special meanings in Internet mail parlance, and both the structure of mail messages and the path that messages take through the mail system are carefully defined. The essential documents are RFC 2821, which defines the Simple Mail Transfer Protocol (SMTP) used to move mail from one place to another, and RFC 2822, which defines the format of mail messages. These RFCs were published in April 2001, updating the original RFCs 821 and 822 published in 1982. (All RFCs are available online at http://www.rfc-editor.org.)

For many years, the only widely used MTA for Unix and Unix-like systems was the venerable sendmail, which has been around in one form or another for 20 years . As a result, many people assume that whatever sendmail does is correct, even when it disagrees with the RFCs or has unfortunate consequences. So even if you're familiar with sendmail (indeed, especially if you're familiar with sendmail), at least skim this chapter so we all can agree on our terminology.