Shared server listed on Backscatterer.org; tracking down the problem

It recently came to my attention that one of my shared server IPs is (are?) listed on Backscatterer.org. Their policy states that they list IPs that are "ending misdirected bounces or misdirected autoresponders or sender callouts".

I have (and always have had) sender callouts disabled for this server, so the problem would apparently lie with one (or more) accounts sending misdirected bounces or autoresponders.

I do see that the mail log has increased in size a lot lately (it used to compress every week or so, but now it's doing it daily), but Mail Relayers doesn't show anything more than normal. Nothing really stands out in the mail log... but I don't really know what to look for, either.

Any suggestions on how I might track this down better? The server management company is unwilling / unable to help.