From couple of weeks ago, all our customers has start receiving spam emails. It seems like spamAssassin is not working as it should work. What could be the reasons?
I really hate when I need to answer my own questions :)
I have added following server list in Plesk control panel -> Server -> Mail -> Preferences Switch on spam protection based on DNS blackhole lists (DNS zones for DNSBL service)
abuse.rfc-ignorant.org; zen.spamhaus.org; cbl.abuseat.org; bl.spamcop.net; nomail.rhsbl.sorbs.net
and It is fixed.