FCrDNS & DNSBL Blacklist Checker
Verify Forward-Confirmed Reverse DNS (FCrDNS) and query 60+ major DNS-based Blackhole Lists (DNSBL / RBL) in parallel to check if your mail server IP address or domain is blacklisted by email providers.
What Is Forward-Confirmed Reverse DNS (FCrDNS)?
Forward-Confirmed Reverse DNS (FCrDNS) is a security verification technique where a server's IP address has a matching Reverse DNS PTR record pointing to a hostname, and that hostname in turn has an A or AAAA record resolving back to the exact same IP address.
Major email providers (such as Google, Microsoft 365, Yahoo, and Spamhaus) reject incoming SMTP traffic or flag messages as spam if the sending IP fails FCrDNS validation (RFC 1912).
What Is a DNS-Based Blackhole List (DNSBL / RBL)?
A DNSBL (DNS Blackhole List) or RBL (Real-time Blackhole List) is a spam prevention database maintained by cybersecurity companies (such as Spamhaus, Barracuda, SORBS, Spamcop, and PASSUR). Mail servers query these lists via DNS to decide whether to accept, quarantine, or reject incoming email connections.
Common Causes of IP Blacklisting & How to Delist
- Compromised User Accounts / Open Relays: Spammers send high volumes of outbound email through unauthorized accounts or open SMTP relays.
- Missing SPF / DKIM / DMARC: Outbound emails fail DKIM signature checks or SPF authorization.
- Shared IP Reputation: Using shared hosting or unvetted cloud provider IP ranges previously used by malicious actors.
- Delisting Steps: Fix the underlying spam source or open relay, verify FCrDNS PTR alignment, and request an official removal request on the specific RBL provider's delisting web portal.