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Real-time blacklists, DNS RBL precisely, are a convenient way to mitigate the SPAM problem on your server despite all critics about it. policyd is another option that you should consider.

If you’re using Postfix as your MTA it’s quite easy to setup RBLs. Simply add the reject_rbl_client option to your main.cf and you’re done.

        reject_rbl_client zen.spamhaus.org,
        reject_rbl_client ix.dnsbl.manitu.net,
        reject_rbl_client bl.spamcop.net,
        reject_rbl_client combined.njabl.org,
        reject_rbl_client list.dsbl.org,

The important question is which blacklists to use. This depends on your preferences, because the lists differ on the IP-ranges they include (e.g. dial-in networks).

I’m quite happy with the ones I’m using of course, but you might find a huge list on the net and a comparison on Jeff’s blacklists comparison