mod_smtpd_rbl brings "RBL" (DNSBL/RHSBL) support to mod_smtpd
-> requires mod_smtpd
-> requires mod_dnsbl_lookup

The only directives introduced by mod_smtpd_rbl are
SmtpWhitelist - Set SMTP whitelist chain name
SmtpBlacklist - Set SMTP blacklist chain name

The main configuration is handled by mod_dnsbl_lookup, see its README for 
a complete description of how to configure multiple chains for DNSBL and 
RHSBL lookups. Once mod_smtpd_rbl knows which chain to use as a whitelist 
and which to use as a blacklist, it will invoke mod_dnsbl_lookup's 
facilities and return information to mod_smtpd via hooks in order to allow 
or deny mail.

Sample configuration for mod_smtpd + mod_smtpd_rbl + mod_dnsbl_lookup

# Enable mod_smtpd
SmtpProtocol On

# Define whitelist and blacklist chains for mod_smtpd_rbl
SmtpWhitelist mywhitelist
SmtpBlacklist myblacklist

# Enable mod_dnsbl_lookup
DnsblLookups On

# The zones and chains for mod_dnsbl_lookup

RhsblZone myblacklist	rhsbl.ahbl.org.		127.0.0.2
RhsblZone myblacklist	abuse.rfc-ignorant.org.	127.0.0.4

DnsblZone myblacklist	sbl.spamhaus.org.	any
DnsblZone myblacklist	cbl.abuseat.org.	any