The name of the system running remote network service
application. For an IP-based application this should be
either a domain name or IP address. For an OSI application
it should be the string encoded distinguished name of the
managed object. For X.400(1984) MTAs which do not have a
Distinguished Name, the RFC 2156 syntax 'mta in
globalid' used in X400-Received: fields can be used. Note,
however, that not all connections an MTA makes are
necessarily to another MTA.