The User Defined SMTP Server Lists section allows for Address Objects to be used to construct a white-list (explicit allow) or black-list (explicit deny) of SMTP servers. Entries in this list bypass the RBL querying procedure. For example, to ensure that you always receive SMTP connections from a partner site's SMTP server, create an Address Object for the server using Add, click the edit icon in the Configure column of the RBL User White List row, and add the Address Object. The table will be updated, and that server will always be allowed to make SMTP exchanges.The System > Diagnostics page also provides a Real-time Black List Lookup feature that allows for SMTP IP addresses (or RBL services, or DNS servers) to be specifically tested.