Organizations use blacklists to limit security threats like spam, malware and viruses. The IP address of a server sending email is pulled from the email’s header and compared to the blacklist.  Anything that originates from an IP address on the blacklist is refused, quarantined or dumped to a spam folder.  Similarly, content of an email is scanned against the domain blacklist.  Any emails from or containing a domain on a blacklist will be dealt with.

Some companies also utilize blacklists to scan inbound or outbound web traffic or to create web or email filtering appliances.  Many companies purchase or utilize multiple blacklists along with their own blacklist information to minimize the potential for spam, malware or viruses passing through their servers.

MxToolbox provides insight into the blacklist reputation of your IPs and Domains.