If the Sender IP 16.42.233.42 Has a PTR that solves to ip42.aaa.it
-and-
ip42.aaa.it doesnt point to 16.42.233.42 there is no A record for PTR record and so Potfix generate a header like this :
Received: from abc.it (unknown [16.42.233.42])
by EFA.gruppocomet.it (Postfix) with SMTP id 3C812A260C
This it a good thing so it will be useful to give a penality to these messages, this is simple :
header FROM_UNKNOWN Received =~ /unknown.+EFA.+/
describe FROM_UNKNOWN Il nome reverse dell Ip server mittente non ha il record A
score FROM_UNKNOWN 0.40
Substitute EFA with your server