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[188.8.131.52]
milter-greylist just takes what it gets and that's often:
milter-greylist: (unknown id): addr [184.108.40.206][220.127.116.11] (...)
Patch is attached. Once applied, clients without a reverse dns mapping
will trigger the SpamAssassin RDNS_NONE test.