Blacklists (RBL/DNSBL) track IP addresses associated with sending spam or suspicious activity. If your IP is on one, your emails may end up in spam, or even be blocked outright by some servers. This tool queries several well-known public lists and tells you if your address is flagged on any of them.
What is an IP blacklist (RBL)?
An IP blacklist (RBL/DNSBL) is a public database that tracks IP addresses associated with sending spam, attacks, or suspicious behaviour. Mail servers check these lists before accepting a message: if the sending IP is on one, the email may end up in spam or be rejected outright.
An IP can end up listed through no fault of your own (compromised machine, dynamic IP previously used by a spammer...). The good news: delisting is almost always possible, directly from the relevant list's website.