A DNSBL (DNS-based blocklist), sometimes called an RBL, is a published list of IP addresses or domains associated with spam or abuse. Receiving mail servers query the list over DNS during a connection: they reverse the sending IP, append the blocklist's zone, and look it up. A returned address means the sender is listed, often with a TXT record explaining why.
DNSBLs matter because a listing can cause your mail to be rejected or sent straight to spam, regardless of how well-authenticated it is. Listings happen for many reasons: compromised servers, shared hosting neighbours, open relays, sudden volume spikes, or spam-trap hits.
DNSBLs operate at the connection or IP level and are separate from DMARC, SPF and DKIM, which authenticate the sending domain. Good authentication and a clean FCrDNS setup support reputation, but they do not remove a blocklist entry on their own.
To check your status, look up your sending IPs and domain against the major blocklists. If you are listed, identify the cause, fix it (secure the host, stop the abusive traffic), then use the blocklist operator's delisting process. Maintaining steady volume, valid PTR records and strong authentication is the best way to stay off these lists.
Check it on your domain
- Blacklist Checker: check a domain or IP against email blacklists.
- PTR Record Checker: reverse-DNS lookup for an IPv4 address.
- Reverse DNS (FCrDNS) Checker: check forward-confirmed reverse DNS (FCrDNS) for a mail IP or domain.