An RBL list is a list of addresses that an RBL list supplier believes are a source of Spam. They can be used with FTGate to prevent machines that are listed in the RBL from sending mail to your server. They can also be used by the Filter section to allow filtering of messages received by SmartPop which passed through an RBL listed site.
Care should be given when selecting which RBL lists should be included because by using a list you are allowing a third party to determine which servers you will allow to send you mail. Many RBL lists contain machines that are called open relays. These relays may or may not be a source of spam but by their inclusion you would block all mail, both legitimate and spam, from that open relay server.
FTGate Technology recommend only the use of Spamhaus lists as they, sbl.spamhaus.org and sbl-xpl.spamhaus.org, do not include open relays.