I’ve been using this [AOL feedback loop](http://postmaster.aol.com/fbl/index.html) system for quite some time. But I guess not many mail admins know about it so I will share it here. AOL feedback loop is a system implemented by AOL on their system that will forward any mail that has been marked as spam by its users back to the administrator of the originating IP.
The following information is copied verbatim from AOL’s page:
Since practically all email lists distributed by spammers include @aol.com addresses, this is a great way to know if your users are sending spam. And you will know that much earlier before you see complaints coming in. Unfortunately email lists distributed by local spammers in Indonesia probably doesn’t have as many aol.com addresses in it, if at all. I wish some popular local ISP or mail provider will implement a system like this. This could be very valuable since local spam is on a very steep rise now.