This chapter covers the output end of the qmail mail system, or how email is delivered by qmail and how it leaves the message queue. The following figure describes this part of the overall structure of qmail described in Chapter 1.
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 ...