Generally you'll want to check SpamHaus to see what the reported issue is:
IP Address 184.108.40.206 is listed in the CBL. It appears to be infected with a spam sending trojan or proxy.
It was last detected at 2012-10-21 09:00 GMT (+/- 30 minutes), approximately 1 days, 7 hours, 59 minutes ago.
It has been relisted following a previous removal at 2011-06-22 17:06 GMT (487 days, 23 hours, 52 minutes ago)
Chances are, there's a compromised server listed on this node that's spoofing an address of yours to send SPAM. You might be interested in checking your logs on your mail server to see if there's any failed responses from hosts that you haven't been sending mail to.