qmail-smtpd does not know if a user exists. Lots of resources are wasted by
scanning mail for spam and viruses for addresses that do not exist anyway.

A replacement smtpd written in Perl could use this module to quickly verify
that a local email address is (probably) actually in use.

This module comes with a daemon program called qmail-deliverabled and a module
called Qmail::Deliverable::Client that copies the entire public interface, but
queries the daemon. Typically, the daemon runs as the root user, and the client
is used by the unprivileged smtpd.