An IP blacklist (DNSBL – DNS-Based Blackhole List) is a database of IP addresses known as spam sources, botnet participants or attack origins. Mail servers and security systems use these lists to block suspicious connections.
If your IP is listed, emails may be marked as spam or rejected. Common causes are a compromised device on the network, a misconfigured mail server or previous abuse of the IP address. Delisting is possible with most providers through their website.