System Service to send mail before system shutdown

I don't think there is an actual System Shutdown notification, but the system will stop your service as part of the process and you could simply send an email whenever your service is being stopped.
However, I would not try to do that unless I really, really have to. You risk timing out because other services your mail feature relies on have been stopped before your service. If your service does not stop within a certain time, the system will simply kill it and the whole thing is useless.

Have you considered alternative behavior?

Soren Madsen