The blacklist test will check a mail server IP address against over 100 DNS based email blacklists. (Commonly called Realtime blacklist, DNSBL or RBL). If your mail server has been blacklisted, some email you send may not be delivered. Email blacklists are a common way of reducing spam.
The Blacklist check queries the major DNS Blacklists - Sorbs, Spamhaus,NJABL, DSBL, CBL and PSBL for specific IP address or hostname and returns whether it is blacklisted for sending spam, as an open proxy relay or for other possibly malicious activities.
A DNS Blacklist (DNSBL), is a means by which an Internet website can publish a blacklist of IP addresses or hostnames. The technology is built on top of the Internet Domain Name System, or DNS which can be easily queried by computer programs on the Internet. DNS Blacklists (DNSBL) are mainly used to publish lists of addresses linked to spamming. Most mail transport agent (mail server) software can be configured to reject or flag messages which have been sent from a website listed on one or more such blacklists.