127 Internet Blacklists: Protection or Guilt by Association?

Part 1 — Understanding UCEPROTECT, Shared IP Addresses, Internet Neighborhoods, and Responsible Security

Digital Life Security Series ...Practical ways to protect your devices, accounts, privacy, information and home

The Internet Needs Protection, But at What Cost?

Every day, billions of email messages travel across the internet. Some are important business communications, personal messages, invoices, and notifications. Others are unwanted advertisements, phishing attempts, scams, and malicious messages designed to steal information.

To help control these problems, internet administrators use systems known as blacklists, increasingly called blocklists.

These systems identify IP addresses and networks associated with suspicious or abusive activity. Email servers can consult them to help determine whether incoming messages should be accepted, rejected, or examined more carefully.

When used responsibly, blocklists are valuable security tools. But what happens when an entire network is identified as suspicious because of the actions of a relatively small number of its users?

And what happens when a perfectly legitimate server gets caught in the middle?

These questions lead us to an important principle: Being associated with a suspicious network does not necessarily mean an individual server is responsible for suspicious activity.

What Is an IP Address Blacklist?

Every internet-connected server uses an IP address. This numerical address allows computers and other devices to communicate across networks.

When a server sends email, the receiving mail system can examine the sending server's IP address and check its reputation against one or more blocklists.

For example, if an IP address has repeatedly sent unsolicited email to spam traps, a blocklist provider may identify that address as a potential source of spam.

A receiving email server might then reject messages from that address, place them in a spam folder, or assign them a higher spam score.

However, blocklists are not all created equally. Their detection methods, listing policies, removal procedures, and accuracy can differ considerably.

Introducing UCEPROTECT

One example worth examining is UCEPROTECT, an organization that maintains several DNS-based email blocklists.

UCEPROTECT uses three primary listing levels, each covering a potentially broader range of IP addresses.

Level 1: Individual IP Addresses

UCEPROTECT Level 1 focuses primarily on individual IP addresses associated with spam activity detected through its reporting systems.

This is the most targeted of its three levels. An individual server may be listed because its address was observed sending unwanted email.

Even here, administrators should investigate carefully. A server could have been compromised, an account could have been abused, or an IP address might have been reassigned from a previous customer.

Level 2: Network-Level Listings

Level 2 takes a broader approach. When abusive activity within an IP allocation exceeds UCEPROTECT's thresholds, the larger allocation can be listed.

This means an innocent server may be included because other IP addresses within the same allocation have generated sufficient spam reports.

Imagine an apartment building containing 100 residents. If several residents repeatedly cause problems, would it be reasonable to deny deliveries to everyone in the building?

That illustrates the concern with broad network-level blocking. The behavior of some can affect many others.

Level 3: Entire Autonomous Systems

Level 3 is even broader. Internet service providers and hosting companies operate networks identified by Autonomous System Numbers, or ASNs.

Under UCEPROTECT's Level 3 policy, all IP addresses assigned to an autonomous system can be included when its spam-related measurements exceed specified thresholds.

A large hosting provider may operate thousands or even millions of IP addresses. Some may host email services, others websites, databases, applications, or unrelated infrastructure.

Being included in a Level 3 listing does not establish that each individual server has sent spam.

Interestingly, UCEPROTECT's own Level 3 documentation acknowledges the possibility of collateral damage and recommends using the list as part of a scoring system rather than automatically rejecting every matching email.

How Big Is Your Internet Neighborhood?

When an IP address is included in a network-level blacklist, the explanation sometimes refers to a problem somewhere in the same internet neighborhood.

But what exactly is a neighborhood on the internet?

Unlike a residential neighborhood, an internet neighborhood does not have a fixed number of addresses. Its size depends on how the network is organized and how broadly the blacklist defines the affected area.

Internet networks commonly use a system called Classless Inter-Domain Routing, or CIDR, to describe groups of IP addresses.

For example, a /28 IPv4 allocation contains 16 addresses, a /24 contains 256, a /20 contains 4,096, and a /16 contains 65,536.

An autonomous system can announce multiple address allocations and may represent a much larger collection of addresses.

Consequently, a network-level listing might include dozens, thousands, or even millions of addresses, depending on the scope of the listing.

That raises a fundamental question: How many innocent servers should be affected because of abusive activity elsewhere within a provider's network?

The answer is not always simple. Hosting providers do have a responsibility to address abuse on their networks. But an individual customer may have no knowledge of, or control over, the behavior of unrelated customers.

The larger the neighborhood, the more important it becomes to distinguish collective network reputation from individual server responsibility.

Is This an IP Blacklist or a Provider Blacklist?

Traditional IP blocklists often focus on individual addresses associated with specific abusive behavior.

Broad network-level systems operate differently. When an entire allocation or autonomous system is listed, the classification reflects the reputation of the larger network rather than the observed behavior of every individual server.

In that sense, Level 3 functions much more like a provider-network reputation system than a conventional individual-IP blacklist.

This distinction matters because customers of a large hosting company may be affected by activity they cannot observe, prevent, or correct.

Some network-level reputation systems encourage affected customers to contact their hosting providers and request improvements in abuse management.

That may be reasonable when a provider consistently fails to control abusive activity. However, suggesting that an otherwise satisfied customer should change hosting companies because of unrelated activity elsewhere on the network raises additional questions.

Moving to another provider does not guarantee that the new network will never experience abuse. It may simply exchange one shared reputation problem for another.

Network reputation is useful information, but it should not be confused with proof that every customer on the network is behaving irresponsibly.

Who Decides Which Networks Are Suspicious?

When a blacklist identifies an IP address or network as suspicious, it is reasonable to ask how that decision was made.

Who collected the evidence? What behavior was observed? How recent is the information? Can the administrator examine the reason for the listing?

UCEPROTECT describes collecting information through spam traps and reporting systems. Its public information discusses several sources of spam-related data, although not every contributing source or underlying observation is publicly identifiable.

That does not automatically make the information inaccurate. Security organizations sometimes limit disclosure to protect detection systems from manipulation.

Nevertheless, transparency becomes especially important when a listing affects many unrelated customers.

Administrators should be able to understand the general criteria for inclusion, how listings expire, and what options exist for correcting mistakes.

A useful technical reference is RFC 6471, Overview of Best Email DNS-Based List Operational Practices.

This document discusses operational practices for DNS-based email lists, including documentation, accountability, and listing and removal procedures.

It provides a valuable framework for evaluating reputation services without focusing criticism on any particular organization.

What Does the “Spammerheavens” Ranking Tell Us?

UCEPROTECT publishes a ranking titled "Current Spammerheavens ;-)" that identifies autonomous systems associated with high spam-related scores.

The ranking includes network provider names, autonomous system numbers, numerical spam scores, and other measurements.

At first glance, the large scores may appear alarming.

However, numbers are meaningful only when we understand how they are calculated, what they represent, and how they relate to the actual number of abusive servers within a network.

A network with a high aggregate score may contain a large amount of abusive activity, but that does not establish that every IP address within the network is abusive.

Nor does a ranking of autonomous systems, by itself, explain why a particular individual server has been included in a broad network listing.

We should distinguish between identifying a network with significant abuse problems and concluding that all customers of that network are responsible.

Statistics can be useful evidence, but only when their meaning and limitations are understood.

A Real-World Experience: From 1&1 to IONOS

Over many years of managing web hosting and email servers, I have encountered situations that illustrate the limitations of IP reputation systems.

My hosting experience goes back to the days when IONOS was known as 1&1.

Throughout those years, I repeatedly observed my server IP addresses appearing on UCEPROTECT Level 2 and Level 3, even though the individual addresses were generally clear of other blocklists I checked.

This was particularly frustrating because I worked to maintain secure servers, monitor email activity, and keep client accounts operating responsibly.

On some occasions, when I received a newly assigned IP address, I discovered that it was already listed by another blacklist provider.

In those situations, I contacted the listing organization, explained that the address had recently been assigned to me, and committed to carefully monitoring the server and its users.

Those providers reviewed the situation and removed the listings.

My experience with UCEPROTECT was different.

When I initially investigated the listings and attempted to resolve them, the removal path presented to me appeared to involve payment. I did not find a practical individual appeal process comparable to those I had experienced with other providers.

That prompted me to investigate further, including reviewing the experiences and recommendations of other server administrators.

I eventually concluded that a UCEPROTECT Level 2 or Level 3 listing, by itself, was not sufficient evidence of a problem with my server.

This is a personal account of my experience, not a claim that every IP address operated by 1&1 or IONOS has always been listed.

It demonstrates why administrators need to distinguish between the reputation of their own servers and the reputation assigned to an entire hosting network.

What If We Applied the Same Logic to Gmail and Outlook?

Consider another example from years of email server administration.

Many phishing messages that have reached my own email filters have appeared to originate from accounts using major services such as Gmail and Outlook.

In my experience, these providers account for a substantial portion of the phishing attempts that successfully pass through my server's initial filtering.

This is a personal observation, not a statistical measurement of phishing activity across the internet.

Large email providers serve enormous numbers of legitimate customers. Unfortunately, criminals can also create accounts, compromise existing accounts, or misuse these services to send deceptive messages.

Some fraudulent messages may be sent slowly or selectively, making suspicious activity more difficult to identify immediately.

Now consider applying the same broad neighborhood logic used by some network-level blacklists.

If some Gmail or Outlook accounts send phishing messages, should every message from those providers receive a poor reputation score simply because it originates from the same general service?

That would potentially penalize enormous numbers of innocent users.

Of course, receiving mail systems can distinguish among sending IP addresses, authenticated domains, individual messages, and other signals. Gmail and Outlook are also large services with substantial abuse-prevention systems.

Nevertheless, the comparison illustrates why collective reputation must be interpreted carefully.

The existence of abusive users within a large service does not establish that every user of that service is abusive.

When a Legitimate Email Account Sends a Scam

There is another important lesson in this comparison.

A phishing message does not necessarily come from a technically suspicious email server.

Criminals sometimes use accounts on legitimate email services. Those messages may pass standard authentication checks because they genuinely originated from the service associated with the sender's address.

SPF, DKIM, and DMARC help verify aspects of email authorization and domain identity. They do not establish that the person sending a message is trustworthy or that the message's claims are true.

Several years ago, one of my clients lost approximately $1,400 after responding to a fraudulent email that appeared convincing.

I routinely encourage clients to contact me when they receive questionable messages, but in this case the message seemed legitimate enough that the client acted without seeking a second opinion.

Experiences like this demonstrate why technical filtering and human judgment must work together.

Even a message from a well-known email provider can be fraudulent. Financial requests, changes to payment instructions, and unexpected demands for sensitive information should be independently verified using a trusted contact method.

No blacklist can replace careful evaluation of what a message is asking us to do.

Should You Pay to Be Removed From a Blacklist?

UCEPROTECT offers paid expedited removal under certain conditions. However, its published policies also describe automatic, free removal when the relevant listing criteria are no longer met.

Therefore, it would be inaccurate to claim that payment is always required for removal.

For an individual server administrator included in a broad Level 2 or Level 3 listing, however, the situation can be difficult. The administrator may have no control over the unrelated servers contributing to the network's listing.

This raises an important question: Should a responsible administrator pay to expedite removal from a list when the underlying activity may be outside that administrator's control?

Several established technology companies have expressed concerns about UCEPROTECT's policies.

Cisco has published guidance recommending against using UCEPROTECT as a blocklist and against paying for delisting.

Proofpoint has similarly characterized the listing policies as unusually aggressive and recommends against paying removal fees.

MXToolbox explains that UCEPROTECT listings can expire without payment and advises administrators to focus on actual email delivery problems rather than assuming every listing requires immediate action.

These are significant industry perspectives, although each organization has its own experience and policies.

They reinforce the importance of evaluating a blocklist rather than automatically treating its findings as authoritative.

When a New IP Address Comes With an Old Reputation

IP addresses are frequently reassigned. A hosting company may recover an address from one customer and later assign it to another.

The new customer might operate a perfectly legitimate website or email server, yet the IP address may still carry a reputation established by its previous user.

This is one reason responsible blocklist management should include procedures for correcting outdated information and evaluating changed circumstances.

It is also why new server administrators should check their assigned IP addresses before relying on them for important email delivery.

A listing may reflect historical activity rather than a current security problem.

What Should You Do If Your Server Is Blacklisted?

Discovering a listing can be alarming, especially for a small business that depends on reliable email communication.

However, a listing is a reason to investigate, not automatically a reason to panic.

1. Identify the exact listing. Determine which blocklist is involved and whether it lists your individual IP address, a larger allocation, or an entire autonomous system.
2. Check actual email delivery. Review server mail logs, delivery failures, and rejection messages to determine whether the listing is affecting communication.
3. Investigate server activity. Look for compromised accounts, unauthorized mail sending, suspicious scripts, or other potential sources of abuse.
4. Review other reputation services. Compare results from multiple reputable sources rather than relying on a single list.
5. Contact the appropriate provider. If the listing involves an entire network, your hosting provider may be better positioned to address the underlying issue.
6. Review removal policies before paying. Determine whether free expiration, correction procedures, or other legitimate options are available.
7. Continue monitoring. Maintaining good server security, appropriate email authentication, and responsible account management helps reduce future problems.

Administrators should also remember that different receiving mail systems use different filtering rules. An IP address appearing on a blocklist does not mean every email provider will reject its messages.

How Should Responsible Administrators Use Blocklists?

Blocklists are most useful when considered alongside other evidence.

Modern email filtering can examine SPF, DKIM, and DMARC authentication, message content, sending patterns, domain reputation, server behavior, and multiple independent threat-intelligence sources.

Rather than automatically rejecting every message associated with a broad network listing, a system may assign a risk score and combine it with other indicators.

This approach helps distinguish genuinely suspicious activity from innocent systems sharing infrastructure with problematic users.

Security decisions should also be proportionate to the quality and specificity of the available evidence.

An individual IP address observed sending spam is not the same thing as an entire hosting network containing some addresses that have sent spam.

Looking Ahead: Could Internet Security Block Legitimate Websites?

So far, our discussion has focused primarily on email. But the underlying issue raises another question.

Home routers, DNS filtering services, and security applications can also use reputation information to identify potentially dangerous websites.

What would happen if these products indiscriminately applied broad email network blocklists to ordinary web browsing?

Potentially large numbers of legitimate websites could become inaccessible simply because their hosting infrastructure belonged to a listed network.

That does not establish that any particular home security product currently uses UCEPROTECT in this way. Nor do we have reliable evidence establishing a percentage of websites that would be affected.

Nevertheless, it is an important scenario to understand, particularly as more households rely on automatic security features built into their routers.

We will explore this subject in Part 2: Home Internet Security: Can Too Much Protection Block the Good?

Final Thoughts: Security Requires Judgment

Internet blocklists serve a legitimate and important purpose. They help administrators identify suspicious activity and reduce unwanted communication.

But their usefulness depends on the quality of their information, the fairness of their policies, and how their findings are applied.

Broad network listings can sometimes penalize responsible users for activity beyond their control. A server can be secure, properly maintained, and operated responsibly while still appearing on a list covering its provider's larger network.

That does not mean we should abandon blocklists. It means we should understand their limitations.

Before judging someone by their internet neighborhood, perhaps we should ask how large that neighborhood is — and whether they even know their neighbors.

A blacklist should be treated as evidence to evaluate, not a verdict to accept without question.

Further Reading

• UCEPROTECT — Official Website and Listing Policies
• RFC 6471 — Overview of Best Email DNS-Based List Operational Practices
• MXToolbox — How Do I Get Off UCEPROTECT?
• cPanel — My Public IP Is Listed in UCEPROTECT
• Cloudflare — What Is an Autonomous System?