Below is a list of some RBLs that can be added.
DNS list Info URL
zen.spamhaus.org http://www.spamhaus.org/zen/
rhsbl.ahbl.org http://www.ahbl.org/documents/rhsbl
truncate.gbudb.net http://www.gbudb.com/truncate/
b.barracudacentral.org http://www.barracudacentral.org/account/register
bl.blocklist.de http://www.blocklist.de/en/api.html#dns
dnsbl.dronebl.org http://dronebl.org/docs/howtouse
dnsbl-1.uceprotect.net http://www.uceprotect.net/en/index.php?m=6&s=10
all.s5h.net http://www.usenix.org.uk/content/rbl.html
ubl.unsubscore.com http://blacklist.lashback.com/
new.spam.dnsbl.sorbs.net http://www.sorbs.net/lookup.shtml
nomail.rhsbl.sorbs.net http://www.sorbs.net/lookup.shtml
zombie.dnsbl.sorbs.net http://www.sorbs.net/lookup.shtml
srnblack.surgate.net https://tools.korumail.com/