First of all, your XBees would need some way of contacting the "outside" world, since an XBee doesn't have IP connectivity inherently (unless you're referring to an XBee Wifi).
What you might want to look at to get your XBee's on an IP network (if not XBee Wifi) is one of our ConnectPort X2, X4, or X2e gateways (depending on the XBee protocol). The ConnectPort has a built-in python interpreter, and can also be added to Digi Device Cloud. This is where your alert capability exists, because Digi Device Cloud supports email notification for various types of Alarms (refer to the Device Cloud User Guide section about Alarms in the Device Management chapter).