DSBL was an open relay/open proxy DNSBL. From the website: "DSBL relied on volunteers who, upon receiving spam, would test the IP addresses that sent them spam for open relay and open proxy vulnerabilities.
"The tests consisted of doing a straightforward open relay test on the sending IP address, as well as open proxy tests on a few well-known proxy ports (1080, 3128, etc), with the aim of relaying a test message to DSBL. Upon receipt of the test message, DSBL would add the IP address to its database."
A noble cause, if perhaps a bit of a manual process. Sad to see it go, though it does sound like perhaps its time has come and gone.
DSBL had DNSBL zone names of list.dsbl.org, unconfirmed.dsbl.org and multihop.dsbl.org.