From LQWiki

RBL stands for Real-time Black-Listing. It is a spam-fighting method used by mail transfer agents such as sendmail to detect that a remote machine is a known spammer, upon receipt of an email from that machine. If the machine is on one or more blacklists the MTA knows about, the email will be rejected as spam.

Adding RBLs to your mail program

sendmail

In your sendmail.mc:

FEATURE(`dnsbl', `bl.spamcop.net', `"Spam blocked see: http://spamcop.net/bl.shtml?"$&{client_addr}')dnl