HP-UX Mailing Services Administrator's Guide (B2355-91064)

If the attempt to return the failed message itself fails, Sendmail returns the message
and transcript to the alias postmaster on the local system. The postmaster alias in
the default alias file (/usr/newconfig/etc/mail/aliases) resolves to root.
If Sendmail is unable to return the message to any of the addresses described previously,
as a last resort it appends the error transcript and returned message to the file
/var/tmp/dead.letter.
Finally, if this fails, Sendmail logs the failure and leaves the original failed message in
the mail queue so that a future queue-processing daemon will try to send it. If this fails,
an error message is returned again.
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.
The Sendmail Utility 35