The blacklist/blocklist check will show if the URL or IP entered is listed with DNSBL or SURBL systems. These systems are used by email system administrators to try and eliminate spam email messages before they reach their users.
DNSBL is a Domain Name Blacklist/blocklist. There are dozens of DNSBLs online, all of which use a wide variety of criteria for listing and delisting addresses. Spam filters utilize various DNSBLs to verify that the incoming emails do not originate from blacklisted sites. DNSBLs are generally the first line of defense against spam.
SURBLs(no longer an acronym) are generally the second line of defense but not all spam filters are capable of using them. SURBLs are blacklists of websites that have been noted for appearing in unsolicited emails. Spam filters that can utilize the SURBLs will scan the body text of emails and extract any website addresses and check them against the blacklists. If an address is found, the message will be flagged as spam and handled accordingly.
Since both methods filter differently they are most effective when combined. Unfortunately as spammers become more clever there isn’t a method that can catch every spam message. It is interesting to note, given the methods spammers use, that DNSBLs tend to change more frequently than SURBLs. If you’re an email system administrator and your server has been blacklisted, you will have to work with the various DNSBLs that may have you listed and/or SURBL (http://www.surbl.org/).