Installing and Administering Internet Services

146 Chapter 4
Installing and Administering sendmail
How sendmail Works
When processing the queue, sendmail first creates and sorts a list of the
messages in the queue. sendmail reads the queue control file for each
message to collect the pre-processed envelope information, the header
lines, and the name of the data file containing the message body.
sendmail then processes the message just as it did when it was
originally collected.
If sendmail detects, from the time stamp in a queued message, that the
message has been in the mail queue longer than the queue timeout, it
returns the message to the sender. The queue timeout is set with the
Timeout.queuereturn option in the /etc/mail/sendmail.cf file
and, by default, is five days.