[Discover] drbcheck: dr. Jørgen Mash's DNS database list checker http://www.moensted.dk/spam/
(antispam test dns lists) 
[Discover] MX Lookup Tool - Check your DNS MX Records online http://www.mxtoolbox.com/ (tools email dns blacklist)
[Discover] Multi-RBL check http://www.robtex.com/rbls/ (spam test useful sysadmin)
[Discover] dnswl.org - Protect against false positives http://www.dnswl.org/ (dns email rbl spam)
[Discover] Barracuda Central http://www.barracudacentral.com/index.cgi?p=lookups (email blacklist spam delivery)
[Discover] Completewhois Multi-RBL Lookup http://www.completewhois.com/rbl_lookup.htm (spam email security network)
[Discover] DNSBL Resource - All About Blacklists http://www.dnsbl.com/ (spam blacklists blogs email)
[Discover] The Spamhaus Project http://www.spamhaus.org/ (spam antispam email resources)
[Discover] SORBS http://www.de.sorbs.net/ (email spam sysadmin free)
[Discover] MX Lookup Tool - Check your DNS MX Records online http://www.mxtoolbox.com/ (tools email dns blacklist)
[Discover] Multi-RBL check http://www.robtex.com/rbls/ (spam test useful sysadmin)
[Discover] dnswl.org - Protect against false positives http://www.dnswl.org/ (dns email rbl spam)
[Discover] Barracuda Central http://www.barracudacentral.com/index.cgi?p=lookups (email blacklist spam delivery)
[Discover] Completewhois Multi-RBL Lookup http://www.completewhois.com/rbl_lookup.htm (spam email security network)
[Discover] DNSBL Resource - All About Blacklists http://www.dnsbl.com/ (spam blacklists blogs email)
[Discover] The Spamhaus Project http://www.spamhaus.org/ (spam antispam email resources)
[Discover] SORBS http://www.de.sorbs.net/ (email spam sysadmin free)