You can also define custom actions and store them in a custom action DLL that is installed on the client computer. Users can create rules using functions from this DLL through the Inbox Assistant. The Inbox Assistant creates a rule on the server mailbox that results in a deferred action. When the client receives a DAM for the action (after the rule has fired), it invokes the DLL that is registered on the client to carry out the action. In other words, all custom actions are invoked as deferred actions. The CRARUN sample application provides an example of this type of custom DLL.
Two Microsoft Exchange Client interfaces are used for custom rule actions: IExchangeRulesExt and IExchangeRuleExtCallback. Both interfaces are defined in EXCHCLI.H.