We are now tracking the Spamhaus The Policy Block list. This list allows service providers to define blocks of IP addresses where e-mail should not originate from.
This service should reduce spam and load on our mail systems.
From the summary at Spamhaus:
The Spamhaus PBL is a DNSBL database of end-user IP address ranges which should not be delivering unauthenticated SMTP email to any Internet mail server except those provided for specifically by an ISP for that customer’s use. The PBL helps networks enforce their Acceptable Use Policy for dynamic and non-MTA customer IP ranges.
Users who have been added to the PBL (most probably by the originating ISP that you receive Internet service from) have recourse; you can go to the page below which will explain how to get unblocked.
More information can be found at this location; http://www.spamhaus.org/pbl/