5/22/2007: This information is out of date. Please click here for my latest take on Spamcop's SCBL.
My most recent take on Spamcop, from February 2007, can be found here. In that commentary, I talk about the history of the Spamcop spam reporting service, its current corporate ownership, and my take on how this type of DNSBL works, especially as to how it relates to to the impact against solicited (wanted) mail.
In February 2007, I found that Microsoft is using Spamcop to filter inbound (corporate) mail. By corporate mail, I mean mail to microsoft.com users, not mail to MSN/Hotmail users. This surprised me, because of what I believe are aggressive listing practices on the part of Spamcop. Indeed, how the issue was brought to my attention was by an unhappy person mad because he couldn't send one-to-one mail to Microsoft, because Spamcop blocked it.
Also, back in 2003, I published an article about the ongoing issues I was having with Spamcop blocking opt-in confirmation requests. Back then I found (through some admittedly unscientific survey techniques) that admins using the SCBL seemed to assume that all blocked mail must be spam because Spamcop blocked it. Not a very encouraging find. It was also a bit insulting to be lectured on how confirmed opt-in worked by people who were blocking confirmed opt-in requests, especially considering I've been pushing senders to implement and utilize confirmed opt-in/double opt-in for many years.