What Is qmail?

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qmail is an Internet MTA for Unix and Unix-like operating systems. An MTA's function is twofold: to accept new messages from users and deliver them to the recipient's systems, and to accept messages from other systems, usually intended for local users.

Users don't usually interact directly with MTAs; they use Mail User Agents (MUAs)—the familiar mail programs such as Outlook Express, Eudora, Pine, or Mutt that users run on their desktop systems. Figure 1-1 shows how all of these agents interact with each other.

Figure 1-1: How the sender, recipient, MUA, and MTA interact

qmail is a drop-in replacement for the Sendmail system provided with most Unix operating systems. What that means is that the user of a system will not necessarily notice a switch from Sendmail, or some other MTA, to qmail. This does not mean that the system administrator won't see a difference. Although all MTAs perform the same functions, they differ widely in installation, configuration, and functionality. Don't assume that your ability to manage Sendmail will let you get up to speed quickly with qmail: It won't. In fact, detailed knowledge of another MTA might even slow you down because you'll be unlearning that system in addition to learning qmail.

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