What this tool does
Queries major DNS-based blackhole lists (DNSBLs) — including Spamhaus, Barracuda, and SpamCop — to check whether a given IP address is currently flagged as a spam or malware source. If you enter a domain instead of an IP, it's resolved to its mail-sending IP first.
Why this matters for email deliverability
Before troubleshooting why legitimate mail is landing in spam or being rejected outright, check whether the sending IP is blacklisted — receiving mail servers routinely consult these lists and will reject or quarantine mail from a listed source regardless of how well SPF, DKIM, and DMARC are configured.
What to do if you're listed
Each DNSBL operator has its own delisting process, usually a web form on their site — search for the zone name shown in the results plus "delisting" to find it. Getting listed again after removal usually means the underlying cause (an open relay, a compromised account, or a misconfigured server sending unsolicited mail) hasn't actually been fixed.