Inter-enterprise messaging system using bridgehead servers

Systems and methods for transmitting messages, such as real-time messages, from an entity outside a firewall to a recipient inside the firewall. The messages include the address of a bridgehead server exposed through the firewall and information identifying the recipient client. The network protected by the firewall can include one or more bridgehead servers, one or more messaging servers associated with each bridgehead server, and one or more clients assigned to each messaging server. When a bridgehead server receives a message, it resolves the address of the messaging server to which the recipient client is assigned. The message is forwarded to the messaging server and, in turn, to the recipient client. The sender of the message does not need to know the identity or address of the messaging server. Moreover, the sender, who may reside outside the firewall, does not need direct access to the messaging servers protected by the firewall in order to establish real-time communication with users inside the firewall. Thus, the messaging systems are compatible with conventional network security measures employed by many organizations. The messaging systems can be used by clients to monitor the online status of other clients or to receive notification of properties associated with other clients.