Whenever 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.