Like IP Lookup comes standard with Dotclear, Out Spam is a filter that checks the IP address of a spammer on shared called DNSBL lists (or RBL) address. The difference with IP Lookup is in the area of intervention: Spam Out prohibits access to spammers and not just comments (cf blog post).
Find a list of servers on this page.
You can add as many servers as you want them separated by commas.
For example: sbl-xbl.spamhaus.org , bsb.spamlookup.net
Do not forget to click Save to save your changes.