postfix sets the hostname of clients without a reverse dns mapping to
‘unknown’ — maybe milter-greylist should do the same:
postfix/smtpd: connect from unknown[184.108.40.206]
milter-greylist just takes what it gets and that's often:
milter-greylist: (unknown id): addr [220.127.116.11][18.104.22.168] (...)
Patch is attached. Once applied, clients without a reverse dns mapping
will trigger the SpamAssassin RDNS_NONE test.