In my position at $current_job, we started to have an issue with blacklisting of email. This was because our listserver of choice (Lyris ListManager), for reasons of performance, accepts all mail for list.domain.com, then later decides if the sender is subscribed, and bounces accordingly. This is a problem because of a thing called backscatter. This is where you address your spam to listservs like ours and forge the recipient. Pretty efficient, if you think about it. This has been totally solved by judicious application of Postfix, Perl, and SQLRelay. So now we have a Postfix server that say 'Hey, you want to send mail to list.domain.com? Awesome, lemme havit!', then passes the mail to a post queue content filter that says 'So you are who? And you want to send mail where? You're not subscribed to that list, I'm gonna drop this on the floor' unless of course you are subscribed to the list then it says 'Awesome! I'll pass your mail along. Thanks!'. Since we have been having record message days and haven't been blacklisted in months I'm gonna have to guess that it all works. Anyone want the source and config, let me know.