LashBack's unsubscribe blacklist ("UBL"), as described by its publisher "is a unique, real-time blacklist of IP addresses which have sent email to addresses harvested from suppression files." It's a neat idea, an interesting way to monitor for failures relating to unsubscribe compliance. But it can be tricky when it comes to things like shared IP addresses, address leakage, subscription forgery or data breaches. Are you for sure blocking the actual sender responsible for failure to respect unsubscribes? As I haven't tested it in a while, I can't speak to its accuracy as far as a spam filtering tool. I did test it years ago but that data is so dusty as to not warrant digging it back up. The blocking list has a long history, I believe it having launched in 2005 or 2006.
Anyway, the reason I mention this today is that I've seen a poster on the Mailop list indicate that the Lashback UBL DNSBL is currently unavailable. I've confirmed that the data links offered up on the UBL website seem to be empty. If you use this DNSBL (ubl.unsubscore.com) in any of your spam filtering tools, you'll want to remove it.