1. A first agent device for an electronic messaging system that includes a plurality of agent devices interacting through exchange of messages, an exchanged message by the first agent device including a content portion and a reference identifier identifying a particular agent device, the reference identifier enabling any other agent device to establish a communication with the particular agent device, the content portion having a type which is either a content sub type or a message-set sub-type, wherein the message-set sub-type allows inclusion of a set of messages or aggregate message sets in the content portion of the message, the plurality of agent devices including a first agent device for message routing with content passing and reference passing, the first agent device comprising: a first interface component configured to load a first agent class specification defining a first finite state machine which is configured to control the function of the first agent device when executed by a process execution engine; a second interface component configured to retrieve at least a subset of a plurality of messages from a message buffer, the plurality of buffered messages originating from a plurality of multi-agent devices linked to a second agent class specification defining a second finite state machine wherein the first finite state machine and the second finite state machine enable the interaction of the first agent device with the plurality of multi-agent devices; and at least one processor component configured to run the process execution engine and, in accordance with the first finite state machine, to process retrieved buffered messages, to generate an aggregate message including at least a subset of the buffered messages in the aggregate message content portion having the message-set sub-type and to provide the aggregate message, in accordance with the first state machine, for further processing to a second agent device, or to another agent operating the first agent device.