The problem that I have is SpamAssassin low detecting rate for spam. It was supposed to be highly increased by using SpamHaus ZEN which I am not sure it is used since detection is too low, so I would like to test with Protected Sky's RBL.
In order to compare the low detecting rate, I have a UTM (Debian based) before virtualmin that uses SpamAssassin to filter spam. UTM is dropping spam messages with score 4 or above, and marking as spam those with score 3 or above. Those marked as spam are reviewed again by Virtualmin, yesterday 1 of 23 was detected again as a spam by Virtualmin (I changed the settings after 10 mails so it may be more accurate to say 1 of 13 or 7% detection rate. Today at 8am from 7 messages marked as spam by UTM, Virtualmin detected only 2, or about 28% detection rate compared to UTM.
Currently my UTM does not allow me to set drop at 3.7 spam level which will eliminate most spam safely with current settings.
Anyway I would like to improve SpamAssassin detection rate. How may I be sure SpamHaus ZEN is used and also how can I add Protected Sky's RBL?