Blacklists are databases with IPs or DNSNames one could want not to accept mail from. There are several different reasons one could find to do that, so there are several lists out there. Some for example of open relays, Sites ignoring RFCs, Urls referenced in Spam emails, and so on. There are several indexes of DNSRBLs out there, for example this one on Wikipedia.
Here is a list of RBLs used by some Citadel operators:* bl.spamcop.net * relays.mail-abuse.org * cbl.abuseat.org * zen.spamhaus.org (former sbl-xbl.spamhaus.org) * dnsbl.njabl.org * dnsbl.sorbs.net * l2.spews.dnsbl.sorbs.net * rhsbl.sorbs.net * bogusmx.rfc-ignorant.org * dsn.rfc-ignorant.org * psbl.surriel.com * blackholes.mail-abuse.org (commercial)
Also check that your own IP address has not been blacklisted! You can do this using DNSstuff, Antispam ufrj, the Sender Policy Framework, or DNS Report.