Trouble with backscatter

Hi All We moved a customer to a new server with clean IP's. Shortly afterwards, one the IP's was blacklisted at backscatterer.org: This IP IS CURRENTLY LISTED in our Database. Please note that this listing does NOT mean you are a spammer, it means your mailsystem is either poorly configured or it is using abusive techniques. This kind of abuse is known as BACKSCATTER (Misdirected Bounces or Misdirected Autoresponders or Sender Callouts). Click the links above to get clue how and why to stop that kind of abuse. I checked Sender Callouts and it's disabled. I haven't checked SMTP logs yet. Is there any other settings I should check that might have gotten this IP blacklisted? Thanks.