HP-UX Mailing Services Administrator's Guide

Mailing Services Overview
The Sendmail Utility
Chapter 138
How Sendmail Handles Temporary Failures
Messages that fail temporarily are saved in the mail queue and retried
later. By default, the mail queue is stored in the directory
/var/spool/mqueue. Sendmail saves the message components in two
files created in the mail queue directory. The message body is saved in a
data file, and the envelope information, the header lines, and the name of
the data file are saved in a queue control file.
Typically, the Sendmail daemon is run with the -q
time_interval
option, as in the following example:
/usr/sbin/sendmail -bd -q30m
In this example, every 30 minutes, Sendmail processes any messages
currently in the queue.
While 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 preprocessed 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.