Qmail uses a very flexible definition of a user. Like most other mail servers, one definition of user is operating-system-defined users. In other words, the users specified in the /etc/passwd file (or wherever the operating system stores user information). These users are used by default for the local domains. However, the operating-system-defined users are easily overridden.

A user, in the most general sense, is a unique delivery script associated with a unique email address. For example, [email protected], [email protected], and [email protected] are all different addresses, and probably refer to different accounts (users): fred, fran, and pat. The simplest case uses the operating system to define all these accounts.

As a category, virtual users are all users that are not defined by the host operating system, but are instead defined either by qmail or by some other program. Qmail has several different forms of virtual users: