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:

@reboot echo "Rebooted, now online!" | Mail -s "`hostname` REBOOT" your@email.com

That's pretty easy now, isn't it?