sendmail gets a SIGTERM signal (as would be the
case if the system were being shut down), it tries to exit cleanly.
First, it unlocks any queued file it is processing. This has the
effect of canceling delivery so that the message will be tried again
when the system comes back up. Then sendmail
resets its identity to the identity it originally ran under. This
causes accounting records to correctly show that the same user
sendmail started as has exited. Finally,
sendmail exits with EX_OK, no matter what, so
that errors will not be produced during shutdown.