sendmail, 4th Edition

Processing a Single Message

A single queued message has a single sender but can have many recipients. When processing a queued message, sendmail attempts to deliver it to all recipients before processing the next queued message.

The first step in processing a queued message is to lock it so that concurrent runs of sendmail do not attempt to process it simultaneously (Current-style file locking on page 398). Then the qf file is opened and read. The sender and all the recipients are gathered from the corresponding S and R lines.

For each recipient, delivery is attempted. If delivery is successful, that recipient’s address is removed from the sendmail program’s internal list of recipient addresses. If delivery fails, that address is either left in the list or bounced, depending on the nature of the error.

After all recipients have been either delivered, bounced, or left in the list, sendmail reexamines that list. If there are no recipients left in it, the message is dequeued (all the files in the queue directory that compose it are removed). If any recipients are left, each recipient results in an M line that is assigned the last error message for that recipient, and the qf file is rewritten with the list of the remaining recipients and a dot. Finally, the qf file is closed, thus freeing its lock.

Under V8 sendmail, the CheckpointInterval option (CheckpointInterval on page 983) causes checkpointing of this process. When this option has a positive value, the qf file is rewritten after that ...