The MAPS (http://mail-abuse.org/) blackhole list doesn't appear to include DNS TXT records -- only DNS A records.
The dnsbl plugin does lookups like this: $sel->add($res->bgsend("$reversed_ip.$dnsbl", "TXT"));
Thus it never finds any entries in a MAPS Blackhole list.
The logic later in the dnsbl plugin is setup to process DNS A records in the query results, so I changed the line above to:
$sel->add($res->bgsend("$reversed_ip.$dnsbl"));
And all is working fine now.
It should probably read: "ANY" rather than "TXT".
