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 unsusbcribe compliance. But it can be tricky when it comes to things like shared IP addresses, addresss leakage, subscription forgery or data breaches. 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.
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. If you use this DNSBL (ubl.unsubscore.com) in any of your spam filtering tools, you'll want to remove it.