[Freeswitch-users] Email notification when user unregistered?

Hi Freeswtich Master, I'm in a situation where I need to proactively moniter whether some device/extension is on in my local network; If for some reason the devices are offline, I need to be noticed immediately so that I can restart the device; Previously I use xlite client to suscribe to the presence notification of the devices, but recently added some devices which not supporting presence notification, so I want FreeSwitch to send an email notification when some user/extention are getting unregistered; How can I capture a Feeswtich user unregister event & launch a LUA script then? Happy days! /Brgds, Alex