It seems a lot of Blacklist filters still use the old DSBL DNS Blacklist, but it's been dead for few years already. Since March 2009, to be exact. However, something changed in the last 24 hours that actually made them go offline.
The result is that any system still using that blacklist sends out bounce messages such as this.
The following recipient(s) cannot be reached: xxx Service unavailable; Client host [x.x.x.x] blocked using list.dsbl.org
If you still have legacy systems running that use the list.dsbl.org blacklist, you may want to remove any references to the DSBL in your configs and stop using that blacklist.