Blocklist (DNSBL) reputation checker
Your marketing emails landing in spam? Often the culprit is a blocklisting — your sending domain or IP is on a DNSBL that mailbox providers trust. Enter a domain and we resolve its mail IPs and check them against 8 well-known, free-to-query blocklists.
How the check works
- 1. Find the mail IPs. From a domain we resolve its A record(s) and its MX hosts' A record(s) — the IPs that actually send and receive your mail. You can also paste a raw IPv4 address directly. Lookups are bounded (a handful of MX hosts and IPs at most).
- 2. Query the blocklists. For each IP we reverse the octets and do a DNS lookup against each blocklist zone — the standard DNSBL protocol (RFC 5782):
4.3.2.1.zen.spamhaus.org. For the domain itself we query URIBLs, which blocklist the domains that appear in spam. DNS only — we make no HTTP requests and store nothing. - 3. Read the answer honestly. A
127.0.0.xanswer means listed (we decode the code to its meaning where the list documents it). NXDOMAIN means not listed. A timeout, SERVFAIL, or refused query means we could not tell— and we never dress that up as “clean”.
Why some zones may say “could not check”
The biggest lists — Spamhaus and URIBL's free tier — refuse queries that come from shared public DNS resolvers and high-volume senders, and Barracuda requires you to register your resolver first. When that happens they don't answer “not listed” — they refuse, and we report that as indeterminate, not a pass. So a green result here means “not listed on the zones we could actually query,” never “not blacklisted anywhere.”
The blocklists we query
6 IP blocklists and 2 domain blocklists (URIBLs). 4 of them may refuse anonymous / public-resolver queries — flagged below, because a refusal is never the same as a clean result.
| List | Zone | Type | Anonymous query |
|---|---|---|---|
Spamhaus ZEN Combined Spamhaus SBL + XBL + PBL — the most widely honored IP blocklist. A listing here is why a lot of mail is rejected outright. | zen.spamhaus.org | IP | may refuse |
SpamCop (SCBL) IPs reported by SpamCop users in the recent past. Time-decayed, so listings expire — good signal for 'sending spam right now'. | bl.spamcop.net | IP | open |
Barracuda Reputation Block List Barracuda's IP reputation list. Effective, but querying it requires registering your resolver's IP with Barracuda first. | b.barracudacentral.org | IP | may refuse |
SORBS (aggregate) Aggregate of SORBS sub-zones — open proxies/relays, hijacked space, and hosts that emitted spam. Broad; can be aggressive. | dnsbl.sorbs.net | IP | open |
SORBS SPAM The SORBS sub-zone limited to hosts observed sending spam to their traps — narrower and less prone to collateral than the aggregate. | spam.dnsbl.sorbs.net | IP | open |
UCEPROTECT Level 1 Single-IP listings for hosts caught spamming UCEPROTECT traps. (Levels 2/3 escalate to whole ASNs and are prone to collateral — we query L1 only.) | dnsbl-1.uceprotect.net | IP | open |
Spamhaus DBL Spamhaus Domain Block List — domains found in spam or with poor reputation. Queried with your domain name directly. | dbl.spamhaus.org | Domain | may refuse |
URIBL (multi) URIBL's combined black/grey/red domain lists. Free tier refuses queries from public resolvers, so treat an empty answer as inconclusive. | multi.uribl.com | Domain | may refuse |
Listed somewhere? A blocklisting usually rides on a deeper authentication gap. Make sure your SPF, DKIM, and DMARC are clean, or run a full email security scan to check all of them at once.
Keep hardening your email
Each record is one layer. Check the rest of yours — every tool is free, no account.
Email security scan
Run all 8 checks at once
SPF builder
Assemble a clean SPF record
SPF analyzer
Count your real DNS lookups
DMARC generator
Build a v=DMARC1 record
DMARC validator
Grade a record you have
DMARC report analyzer
Read a rua= XML report
DKIM validator
Check a DKIM key
Header analyzer
Trace a suspicious email
BIMI check
Get your logo in inboxes
MTA-STS
Force TLS on inbound mail
TLS-RPT
Get TLS failure reports
DMARC visibility
Ingest your rua= reports
Continuous monitoring
Catch record drift on a schedule