>
> Yes, have your ISP show you the exact messages they are
> complaining of. If they have detected a problem, have
> them share that evidence with you so you can see exactly
> what's going on.
>
The OP indicated he knows his server exhibits the bad behavior his ISP is complaining about. The exact message is irrelevant if he knows his system accepts then bounces spam. That is a broken behavior.
Spamhaus has a good explanation.
http://www.spamhaus.org/faq/answers.lasso?section=ISP%20Spam%20Issues#109
"So-called "spam firewalls," software running in front of production servers to process out spam and viruses, can be a problem for other networks if they simply deflect the spam on to other mailboxes. Most spam and all mail-borne viruses use a forged Sender address, and bouncing it back to that address only results in sending unwanted and burdensome mail to innocent third parties. Either reject the SMTP connection with a 5xy message, silently discard it (your firewall identified it as spam, remember?), or file it in a quarantine area for *your* users to glean. Don't make it someone else's responsibility when they are almost certainly not involved."
--
Dan Kriwitsky