DNSBL Blacklist Check

DNSBLs, the honest picture

A DNS-based blocklist (DNSBL, sometimes called RBL for Real-time Blocklist) is a mechanism for publishing lists of IP addresses considered problematic for email sending. Mail receivers query these lists during SMTP transactions to decide whether to accept, reject, or flag messages from specific sending IPs. The mechanism has been around since the 1990s and remains a core part of email deliverability infrastructure in 2026.

The tool queries six major public DNSBLs in parallel and tells you if the IP you're checking is listed on any of them. This is one of the first checks I run when investigating a mail deliverability problem — being listed on even one of these can significantly hurt delivery to major mailbox providers, and knowing which list has you flagged is the first step to getting it addressed.

The blocklists this tool checks

SpamCop

Aggregated from reports submitted by SpamCop users. Listing indicates that recipients have reported the IP for sending unwanted mail. SpamCop is a widely-used list; listing here affects delivery to some major providers.

Barracuda

Maintained by Barracuda Networks, a commercial email security vendor. Listing means their systems have identified the IP as a source of unwanted mail. Their subscribers use this list; listing affects delivery to organizations that use Barracuda's email security products.

DroneBL

Focuses on hosts that have been observed acting as parts of botnets — compromised systems being used to relay spam. Listing here strongly suggests the IP is either compromised or being used by someone who is.

Mailspike

Tracks bulk mail senders and known spammer IPs. Widely used by mail providers. Listing affects delivery meaningfully.

PSBL (Passive Spam Block List)

Automatically-generated list of IPs seen sending spam. Automated methodology means both fast reaction to new spammers and potential for false positives, so there's a well-defined delisting process.

UCEPROTECT-1

The most aggressive of the major public lists. Lists IPs that have sent any unsolicited commercial email, regardless of context. Legitimate senders sometimes end up here through bulk mail from an IP range that also sends spam. Delisting is possible but delisting fees are contentious in the industry.

Why Spamhaus isn't checked

Spamhaus is the largest and most influential of the DNSBLs. It's also the most restrictive about who can query it. Spamhaus explicitly forbids public free tools from querying their lists on behalf of arbitrary users — they require paid subscriptions for any tool that lets end users check IPs on demand. This tool respects that policy and doesn't query Spamhaus.

If you need to check Spamhaus for a specific IP, they offer their own lookup page at spamhaus.org where you can check individual IPs manually. For automated integration, a Spamhaus data subscription is required.

How to interpret results

Not listed on any of the six

Good sign but not a complete picture. Being clean on these six doesn't guarantee clean everywhere. If you're still having deliverability issues, check Spamhaus manually, look at the sender authentication configuration with the Email Health Check, and consider whether reputation history (from before you took ownership of the IP) might be affecting current delivery.

Listed on one or two

Real problem but usually solvable. Each list has its own delisting process. Some (PSBL, DroneBL) have automated delisting for IPs that stop misbehaving. Others (SpamCop) require you to identify and fix the underlying issue before you can request delisting. Others still (UCEPROTECT) have more contested delisting procedures.

Listed on multiple

Serious problem indicating either a compromise, a purchased-list issue, or a genuinely spammy sending pattern. Investigate the underlying cause before working on delisting — otherwise you'll get relisted quickly.

Listed but you don't send mail from that IP

Common for IPs that used to belong to spammers and have been reallocated. New tenant of the IP inherits the reputation. Delisting is usually possible with a clear "IP was reassigned, we don't send bulk mail from it" case. If you're a business that just got a new IP allocation from your ISP, running this check preemptively is worth doing.

What "listed" actually does to your mail

Different receivers use DNSBLs differently. Some use them for hard rejection — mail from a listed IP gets refused at the SMTP handshake. Some use them as scoring signals — listing adds to a spam score but doesn't automatically reject. Some use them for flagging — mail gets delivered but marked as suspicious.

Major mailbox providers (Gmail, Outlook, Yahoo) don't publicly disclose which DNSBLs they check or how they weight them. Their internal systems are informed by public lists but not directly dependent on any single one. Being listed on a major public DNSBL correlates with worse delivery to major providers, but the causation is indirect.

Smaller corporate mail systems often use DNSBLs more literally. If a customer's internal mail rejects your mail with "IP is on SpamCop," that's a direct DNSBL-driven rejection.

Common reasons IPs end up on blocklists

Compromised server

A server that got hacked and is being used to relay spam ends up on multiple lists quickly. First step in remediation is finding and closing the compromise. Delisting requests without addressing the underlying compromise result in immediate relisting.

Poorly-configured mail server

Open relays (mail servers that accept mail from anywhere and forward it) are exploited by spammers within hours of going online. Any mail server should be configured to only accept mail from authenticated users or from allowed source networks. Open relay configuration was common in the 1990s and is now considered a serious misconfiguration.

Bulk mail from a shared IP range

Some cloud providers assign IPs from ranges that other customers have used for spammy purposes. Even legitimate senders on those IPs can inherit reputation problems. Choosing a hosting provider with a good reputation for the IP ranges they allocate matters.

Legitimate bulk mail with a poor list

Sending bulk marketing mail to lists with high bounce rates or high complaint rates results in listing. This is why legitimate marketing platforms invest heavily in list hygiene, engagement tracking, and gradual sending to warm up new IPs.

Corporate mail from residential IP

Consumer residential IPs are on many blocklists specifically because legitimate mail servers don't run on them (residential ISPs typically block outbound port 25). Trying to run a mail server on a residential IP results in listing on residential-focused blocklists regardless of actual behavior.

Newly-allocated IP with prior owner's reputation

As mentioned above. Delisting is usually straightforward when this is the cause.

Getting delisted

Every list has its own process. Common patterns:

Automated delisting

Some lists (PSBL, DroneBL) will delist an IP if the underlying behavior stops. Fix the compromise, stop sending spam, wait for the observation window, and the listing drops off automatically.

Request-based delisting with proof of remediation

Most lists have a form or email process for requesting delisting. You explain what caused the listing, describe what you've done to fix it, and they review and delist if satisfied. Response times vary from hours to weeks depending on the list and their backlog.

Fee-based delisting

UCEPROTECT specifically has a fee-based expedited delisting option that's controversial in the industry. You can pay to delist quickly or wait for the natural delisting timer. The pricing model has been criticized as coming close to extortion; many mail admins refuse to pay on principle.

Waiting for the listing to age out

Some lists automatically delist after a period if no new behavior is observed. Time-based delisting rewards operators who fix problems and don't repeat.

What DNSBLs can't tell you

Why you're actually in the junk folder

Being clean on DNSBLs doesn't mean your mail lands in inbox. Reputation, content, sender authentication, and provider-specific rules all matter. DNSBL status is one signal.

Whether major providers are blocking you specifically

Gmail's, Outlook's, Yahoo's internal reputation systems aren't public DNSBLs. If you're in Gmail's spam but clean on public DNSBLs, you might have a Gmail-specific reputation problem that isn't visible externally.

How other lists that aren't checked here rate the IP

The six lists checked are widely-used but not comprehensive. There are dozens of other DNSBLs. Being clean on these six doesn't mean clean everywhere.

Related tools

Use this tool alongside the Email Health Check for domain authentication configuration and the Email Spam Score for message content evaluation. All three together cover most of what you can check without receiver-specific reputation data.

To identify the network operator of a specific IP (useful when the listed IP isn't yours and you want to know who to contact), the IP Lookup tool shows ASN, ISP, and geolocation.

Privacy

DNSBL queries go directly from the Cloudflare Worker to public DNS resolvers. The queried lists see the queries but don't retain per-query records for public lookups. Nothing beyond the IP you're checking is transmitted, and no history of your checks is retained on the myipcat.com side.