Check if an IP address is listed on major DNS blacklists
DNSBL (DNS-based Blackhole List) and RBL (Real-time Blackhole List) are lists of IP addresses known for sending spam or malicious traffic. Mail servers check these lists to filter incoming email.
IPs get blacklisted for sending spam, hosting malware, being part of a botnet, or having compromised security. Shared hosting IPs can be blacklisted due to other users on the same server.
Each blacklist has its own delisting process. Generally you need to fix the underlying issue (stop spam, patch vulnerabilities), then request removal through the blacklist provider website.