What is a DNSBL, and why should it be checked?
A DNSBL (DNS-based Blackhole List), often called a spam blacklist, is a shared list of IP addresses and domains that mailbox providers have flagged for sending spam, malware, or abusive traffic. If a mail server's IP or a sending domain ends up on one of these lists, delivery rates can drop sharply – messages get silently filtered to spam or rejected outright, often with no warning to the sender.
What this tool checks
Enter a domain, an IPv4 address, or an email address to check it against major public blacklists, including Spamhaus ZEN, Spamhaus DBL, Barracuda, SpamCop, SORBS, PSBL, CBL and SURBL. When a domain is entered, its resolved IP addresses are also checked against the IP-based lists, since a domain and its mail server can each be listed independently.
What to do about a listing
- Confirm the cause – most lists publish a reason, such as reported spam volume or an open relay.
- Fix the underlying issue first (secure an open relay, stop a compromised account from sending, clean a purchased list) before requesting removal.
- Use each list's own delisting process – most are free and take anywhere from a few hours to a few days to process.
Since a shared host IP can be listed because of another tenant's bad behavior, it is worth checking periodically even without an obvious deliverability problem.
