Message delivery approaches in which senders define filters with associated actions for evaluation in relation to specified messages. After creating and storing filters with specified actions, senders dispatch messages to a processing system, which evaluates the filters against the messages. If a match occurs, the processing system performs the specified actions on the messages. The processing system may send the same message multiple times to different receiving systems, and may modify the source IP address and outbound interface of the message for each receiving system. The source IP address or interface may be modified by a filter in response to external events, such as a receiving system blocking another copy of the message. A single message processing system can appear as a large number of virtual message sendingunits.