The following diagram is a part of the overall structure of qmail described in Chapter 1.

The only way of getting email into the on-disk queue in qmail is to use the qmail-queue program. This is a simple program with a single task: take an email and store it in the on-disk queue.

Any program can use qmail-queue to inject a message into the queue, provided it can present the email message and the envelope information (the sender and the list of recipients) in the form that qmail-queue requires.

The on-disk queue is somewhat complicated. It is the communication and storage mechanism used for handing messages to the qmail-send component. The qmail-queue program handles the details of injecting messages into the queue, just as the qmail-clean program handles the details of removing messages from the queue. It is often tempting to attempt to manipulate the queue directly—for example, to remove a message from it. Naïvely modifying the queue, however, is a quick...