DNSBLs are a good idea, and have really helped reduce the amount of spam received by our servers here at work, but their effectiveness is constantly being eroded by lazy organisations and companies. Over the past couple of weeks I've had to remove 2 of the better known DNSBLs from our server configuration, simply because customers and/or clients have got themselves blacklisted and have no idea how to get removed. As far as they're concerned, the problem is with us because we're the ones rejecting their messages,
People are all too happy to sit around and complain about spam, and say they're willing to do anything to fight it, and yet here are perfect real world examples where people are presented with that opportunity, and prove that they really aren't that interested at all. The thing that gets to me the most is when our company emails the blacklisted people and says "Yes, the problem was at our end". The problem is most definately NOT at our end, the problem is you getting your mailserver blacklisted.
Recently I've had to remove list.dsbl.org, and SORBS (http://www.dnsbl.au.sorbs.net/) from our configuration (although I decided to add china.blackholes.us and nigeria.blackholes.us in there to compliment russia.blackholes.us) - SORBS is an interesting one, as the criteria to get on there seems somewhat loose, and the means to get off it are slightly harder. At one point it seemed that a large number of NZ ISPs were listed in SORBS, making the service somewhat unusable. It's somewhat frustrating to have to disable a useful tool, but there really is no other alternative, and it's very similar to when I had to disable "Verify senders Domain in DNS" about 8 months ago because a company we deal with regularly had some weird malformed MX records which wouldn't resolve properly, and hence the verification failed..
Bring on the spam I guess..