What is a DNS blacklist (DNSBL)?
DNS-based blocklists (DNSBLs, also called RBLs) are lists of IP addresses known for sending spam. If your mail server's IP is listed, your email may be rejected or filtered. IPeek checks your IP against a curated set of widely trusted blocklists, in real time, and shows which have listed it and why.
How DNSBL lookups work
A DNSBL works through DNS itself. To check an IP, a mail server reverses the address and queries it as a hostname under the blocklist's zone — for 198.51.100.10 against a list at example.dnsbl, it queries 10.100.51.198.example.dnsbl. A DNS answer (typically a 127.0.0.x address) means the IP is listed; no answer means it's clean. The return code often encodes the reason for listing. IPeek runs these lookups in real time across multiple trusted lists and aggregates the results.
Why an IP gets listed
IPs land on blocklists for concrete reasons: sending spam or high volumes of unsolicited mail, a compromised server or malware-infected machine relaying spam, a misconfigured open relay, hitting spam traps (addresses that should never receive mail), or sharing a neighborhood with bad actors on a poorly managed network. New or recently reassigned IPs sometimes inherit a prior owner's bad reputation. IPeek shows which lists flagged your IP and, where available, the reason so you can address the root cause.
How to get delisted from a blocklist
Delisting starts with fixing the cause — close open relays, patch compromised machines, stop the spam source, and authenticate your mail with SPF, DKIM, and DMARC. Then visit the specific blocklist's website and submit a removal request through its delisting form; most maintain self-service tools. Some lists expire entries automatically once spam activity stops, so clean behavior over time can clear you. Requesting delisting without fixing the underlying problem usually leads to a quick relisting.