Users on the Mailop mailing list are reporting that Webiron (all.rbl.webiron.net) blocklist appears to be malfunctioning. Its domain has expired and the temporary holding pattern pending payment or termination has resulted in the Webiron DNSBL effectively "listing the world" because of wildcard DNS entries.
Another list, BSB (bsb.spamlookup.net), a DNSBL focusing on "comment spam," also recently appears to have died, as reported by MX Toolbox back on April 17th.
When most lists "die" or malfunction, they often end up with wildcard DNS entries in place, as this is a common domain DNS setting implemented by registrars, domain speculators, or domain parkers. What this means is that every single DNSBL query made to the DNSBL's domain is falsely returns with "yes, block that IP address." Meaning your spam filter suddenly blocks 100% of your inbound mail. This is bad news, if you like to actually receive inbound mail.
If you're using either of these lists, you should cease doing so immediately, as their use may impede your ability to receive inbound mail successfully. As always, it's important to pay attention what DNSBLs you use for spam filtering, and periodically review and ensure that they still exist and that they're working properly.