The Domain Name Used for Cluster parameter is closely associated with the Host Name used for Cluster parameter. It is used to override the default domain used in messages originated by the mail relay. Usually, you change this parameter for the same reasons that you would change Host Name used for Cluster.
Enter the desired legal (though not necessarily fully-qualified) domain name in the text box.
ZixCorp recommends that you explicitly set the value of this parameter to the name of your top-level domain (for example: zixcustomer.com). This ensures that all of the ZixGateway appliances in your cluster are acting in concert with regard to both the outside world and other internal systems.