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Using SORBS with Postfix.
Two parts of the configuration need to be changed:

In your main.cf file (usually /etc/postfix/main.cf) set the following:
  1. maps_rbl_domains = dnsbl.sorbs.net
  2. smtpd_client_restrictions = reject_maps_rbl

Note: the 'reject_maps_rbl' restriction should be added rather than replacing the current options.