A facility for providing extensible alert types is described. The facility includes an endpoint component that has an associated application, an alert handler component provided by the application for handling received messages and providing alerts to the application, and a message transport component that receives a message, determines whether the message contains an alert, locates an alert handler to handle the alert, and provides a portion of the received message to the alert handler component so that the alert handler component can handle the alert. The facility can receive a request to register an alert handler, add an indication of the alert handler in a mapping data structure and, upon receiving a message containing an alert, identify an alert handler to handle the alert contained in the message.