In March 2005, I was helping a top-tier ISP with an issue related to a sender utilizing co-registration. The sender wasn't a client of my (then) employer, but the ISP had asked me to assist them with an issue. When you signed up on this sender's site, if you opted-in, your data was passed to a co-reg vendor for various other purposes. This process was supposed to be all double opt-in. It clearly wasn't. At various stages in testing the processes, I had given them unique addresses, so I could test the system without appearing to be a repeat visitor. I shared info with the ISP (who passed it on to the sender) on what process flaws I found. After some go arounds with everybody involved, address and consent verification was properly enabled. I called it a win and moved on to my next fire of the day.
Moving on to today, October, 2006. I'm looking through one of my big "spam" folders. I note that from August 31 to October 6, I've received 549 email solicitations to one of the addresses that was passed along to the co-reg vendor. That single email address has received 14 advertisements every day, for as far back as I have data handy. It suspect it goes back further, but I don't have any data to determine this.
I'm not looking very deeply at the messages themselves to see if they're CAN-SPAM compliant. They probably are. That's not really the issue. My focus here is more on the volume of messages being received: More than 500 in just about five and a half weeks. That's very excessive.
Think about it. If you're a savvy email marketer, you know about the concept of email frequency. How often you should mail your list. Smart strategists spend a lot of effort testing to determine what mailing frequency will get you the best click through and conversion response rate. But if you buy a co-reg list, you don't know who else is mailing it and how often. You can control your frequency, but you can't control everybody else's frequency. So you have no idea if the list is being burnt out through excessive mailing (as it clearly is in this case), or even if the other senders messages are appropriate and non-objectionable. Your mail becomes just one of the potentially many messages the people on the list are receiving. Are you sure these people are going to respond well? I'm not.
Just another data point on why buying co-reg lists from a list broker isn't that great of a practice.