The rbl feature was introduced in V8.10 as an aid to blocking spam email. But because it directly looked up hosts at rbl.maps.vix.com, it was soon rendered obsolete. V8.11 sendmail replaced this rbl feature with the dnsbl feature (Section 7.2), which allows you to specify the host to use for lookups. V8.12 sendmail extended that ability further with the enhdnsbl feature (Section 7.2.2), which also allows you to customize error messages and determine what to do with temporary failures.