Real Time Blacklists
RBL's are lists of IP addresses which are known to send SPAM.
How RBL's Work
RBL's work through DNS queries. Mail servers perform a dns lookup of the senders IP address through a RBL (such as zen.spamhaus.org). If the senders IP is listed in the RBL, then your mail server can perform an action (usually Bounce the Message)
By implementing RBL's you are giving a 3rd party control over your mail server. If the RBL mistakenly lists the wrong IP then legitimate mail could get bounced/rejected. Before implementing RBL's make sure the RBL is trustworthy and builds its lists based upon your needs. There are many RBL's available, each doing different things. Some build their lists based upon Dynamic IP ranges, others based on viruses, and others based on "Spam Traps". The bottom line is be careful!
Postfix RBL Controls
To perform RBL checks in Postfix UCE Controls use reject_rbl_client rbl.provider under smtpd_client_restrictions'
smtpd_client_restrictions = reject_rbl_client sbl-xbl.spamhaus.org
This checks the senders IP early on, saving system resources.