qmail

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.

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 ...