+-----------------------------------------------------------------------
| DNSBL-list filter setup
+-----------------------------------------------------------------------

filter-dnsbl looks up the IP address of the sender at the blacklist
(a domain name) and by default drops the connection if it is found.

To use the DNSBL-list filter, you must declare it first in your OpenSMTPD 
configuration.

Edit the @PKG_SYSCONFDIR@/smtpd/smtpd.conf file to declare the filter:

  filter "dnsbl" \
  	 proc-exec "@PREFIX@/libexec/opensmtpd/filter-dnsbl [options] <blacklist>"

The following options may be passed to filter-dnsbl:

-m  allows the message to continue, but it will be
    marked with an X-Spam and X-Spam-DNSBL header.

-v  more verbose logging. 

Then add the filter to each listener that should be filtered:

  listen on socket filter "dnsbl"
  listen on $ipv4 [...] filter "dnsbl"