You want to be notified by email every time that one of your servers reboots, for whatever reason that might be.
The most straight-forward way to accomplish this is by using the
cron service. You want to fire up the following command:
$ crontab -e
and then add the following statement all on a single line:
$ crontab -l | grep reboot @reboot echo "Rebooted, now online!" | Mail -s "`hostname` REBOOT" email@example.com
That's pretty easy now, isn't it?