The primary function of the on-disk queue is to serve as a reliable storage and signaling mechanism for the qmail-send program, which is the heart of the qmail queuing system. The qmail-send program's most fundamental task is to make the primary routing decision: whether a given email should be delivered locally or remotely. This decision is made exactly once per recipient, and is stored in the queue with the email.

The qmail-send program can be thought of as a military general, commanding the qmail delivery army. As a general, it has two sergeants: qmail-lspawn and qmail-rspawn. Depending on whether a given email should be delivered locally or remotely, delivery commands for that email are given to either qmail-lspawn (for local deliveries) or qmail-rspawn (for remote deliveries). Like qmail-send, these two programs make key decisions and then delegate responsibility to the foot soldiers of the qmail delivery army: qmail-local and qmail-remote. The qmail...