Check if a domain or IP address is listed on any email spam blacklists (DNSBLs).
A DNS Blacklist (DNSBL) is a real-time database of IP addresses or domains known to send spam or engage in malicious activity. Email servers query these lists to decide whether to accept or reject incoming mail.
Each DNSBL has its own delisting process. Typically you need to resolve the issue that caused the listing (e.g., clean a compromised server or fix email practices) and then submit a delisting request directly to the blacklist operator.