One of the most interesting extensions to the standard mail setup discussed in earlier chapters is that of virtualization. There are many reasons for wanting to virtualize email services, from hosting multiple domains with different users to simply extending the ability to apply policies to different sets of email. There are three basic techniques that are used with a standard qmail system for attaining different forms of virtualization: qmail's control/virtualdomains file, user-definable address extensions, and running multiple qmail instances on the same system.
The most straightforward mail handling in qmail is used for what are known as the local domains: those listed in the control/locals file. The users for these domains are all the same, and are typically the users defined in /etc/passwd, though they can be defined in users/assign as well (discussed in the Non-Virtual Non-System Users section). Qmail, however, has another sense...