On Fri, Jun 17, 2011 at 10:46:53PM +0200, m...@smtp.fakessh.eu wrote:
> postscreen_dnsbl_sites = cbl.abuseat.org*2, bl.spamcop.net*1, 
> b.barracudacentral.org*2 ,zen.spamhaus.org*3 ,sbl.spamhaus.org*2,
> xbl.spamhaus.org*2
> postscreen_dnsbl_threshold = 2

You have no fewer than four useless lookups in the above. A Zen 
lookup includes SBL and XBL, and XBL includes CBL. This is all 
unnecessary load on the DNSBL nameservers, and gains you nothing.

With postscreen_dnsbl_threshold = 2, your Spamcop lookup is also 
useless. If any other DNSBL returns positive, your threshold score 
will be reached. And there are no other 1-point lookups that could 
join with Spamcop and reach the threshold.

What you have in the quoted lines is functionally equivalent to:
    postscreen_dnsbl_sites = b.barracudacentral.org,zen.spamhaus.org
leaving postscreen_dnsbl_threshold at the default value of 1.

Soon I will be putting up a web page with slides and notes for my 
postscreen talk at Southeast (USA) LinuxFest. In the meantime, you 
can see what I have posted about my postscreen config here:
    http://permalink.gmane.org/gmane.mail.postfix.user/218114
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