A pure MAPI solution should use PARAMBLK to signal both agents. The PARAMBLK can be used to add additional features. The host agent may be launched by a message that permits multiple agents to be executing concurrently. Multiple agents may be both a blessing and a curse. Multiple agents may permit faster turn-around, but run risks of blockage and running out of resources unless limited. The scenario of a financial system after a mail server has been down for a day could overwhelm the system.