What Is an IP Blocklist and How To Check if Your IP Address Is Blocklisted 

7 min read
16 September 2021
Gustavas Davidavičius

How does an IP address gain a bad reputation, and how are disreputable IPs filtered to ensure network safety?

A warning sing on a computer with the shield above it.

An IP blocklist is a mechanism put in place to check for poor reputation and malicious IP addresses. IP reputation signifies whether the device connected to the IP address is trustworthy or not. An IP blocklist can help figure out whether the resource is associated with, for example, spammers or cybercriminals.

IP blocklisting, formerly known as IP blacklisting, is a kind of filtering system used for anti-abuse purposes. If an IP address ends up in a blocklist, it no longer can access the network. For example, if an email sending IP is blocked, all emails sent from it are bounced and cannot reach the recipient. 

This report sheds light on how blocklists work, how IPs get flagged, and how to delist IP addresses from blocklists.

Why does an IP address get blocklisted?

Typically, you would have an IP address blocklisted due to illicit behavior. However, if your IP was blocklisted, that doesn’t necessarily mean you have done anything wrong. Perhaps someone is maliciously exploiting your IP address, or you inherited an IP address that was already included in some IP blocklist. 

In any case, if an IP address has been added to an IP blocklist, it’s likely to be related to one of the common reasons:

  • Email recipients mark received messages as spam
  • Spam is sent on purpose, or daily sending limits are abused
  • A mailing server is hijacked to send spam or malicious emails en masse
  • A domain is hijacked by cybercriminals who perform illegal activities
  • Someone on the network has a malware-infected device
  • Questionable software runs on the device
  • The IP address is associated with a questionable website
  • The IP address was used in an undesirable way by a previous user

Blocklists and spam

Spam is an issue of epic proportions, and statistics show that 45% of all sent emails are spam. Naturally, a great deal of IP blocklisting is related to spam. 

Spam is an unsolicited message that usually delivers promotional content. In some cases, recipients might accept spam willingly, as they subscribe to suspicious newsletters and emails promoting services. In general, a spam message is classified as mail sent without permission.

When recipients receive unwanted messages, they can mark them as spam. When enough recipients flag mail servers by producing spam complaints, they can be blocklisted, resulting in active IP blocklisting and bounced emails. In this situation, the sender’s IP address is flagged, and the internet service provider’s (ISP’s) spam filters do not let the message through at all. 

A mail server can also be hijacked to send spam and emails containing malicious links or files illegally. In a situation like this, it is crucial to regain control and secure the server as soon as possible. Unfortunately, a mail server can be hijacked, and even government-sponsored attackers may get involved

Image showing emails filtered into Inbox and Spam boxes.
IP blocklisting is often related to spam

Other blocklisting reasons

Your IP can also be blocklisted if it is associated with an infected device. When malware invades your device and you continue to move around the web, networks and websites might recognize your IP address as potentially dangerous. In this situation, removing malware and securing the device is the first order of business.

You may find an IP blocklisted even if you run suspicious software without intending to use it maliciously. Questionable web browsers, for example, could get you in trouble, which is why you need to pick and choose what kind of software you run on your device carefully. 

Needless to say, if you run a site that is associated with illegal activity, such as exposing visitors to phishing scams or distributing illicit material, visitors will be blocked from accessing it, and the IP address associated with the site will be added to IP blocklists as well. 

Finally, you may inherit an IP address that has been added to an IP blocklist due to the actions of the previous user. Internet Service Providers assign new users IP addresses that other users have previously used. If these users were exploiting the resources in malicious ways, you might inherit a poor reputation IP address due to someone else’s behavior.

Different types of IP blocklist

An IP blocklist contains IP addresses that have been marked as potentially unreliable due to suspicious or malicious behavior. Different types of IP blocklists include:

  • Email-based blocklists
  • Domain Name System/DNS-based blocklists
  • Phishing-based blocklists
  • Malware-based blocklists

Email and DNS BLs are intertwined. An email blocklist acts as a spam filter and ensures that potentially malicious or spam emails do not reach the recipient. A DNS-based blocklist works with the domain names associated with the IP addresses that may be related to spam and potentially harmful emails. 

A graphic showing how emails are filtered to check for spam.
How spam filtering works

In the same regard, phishing BLs are intertwined with malware BLs. Tools like Google Safe Browsing, PhishTank and OpenPhish provide such blocklists, and they were developed to detect phishing and malware-related activity on websites. If a website is blocklisted due to malicious activity, these databases notify the webmasters of the flagged sites about the potentially impending IP blocklisting. 

What is DNSBL?

DNSBL stands for DNS blocklist or Domain Name System blocklist and works as a spam filter set up to block messages coming from specific IP addresses. The concept of DNSBL was introduced in 1997 by Dave Rand and Paul Vixie.

Once there’s a record of spam being sent from a specific domain, the server is automatically added to the blocklist. This results in all sites relying on the blocklist to bounce messages coming from the flagged server. So, for example, if you are an email system administrator, you will need to work directly with the DNSBL to get delisted. 

Multiple DNSBL sites exist, and independent spam filters use various DNSBLs to cover all grounds to ensure that unsolicited emails and emails that may contain malicious content are bounced.

Ultimately, most DNSBLs assist with spam-related blocklisting. That said, some blocklists also look for malware, phishing and other types of cyber threats.

A graphic showing the mechanism of a DNS blocklist.
DNSBL filtering mechanism

What is SURBL?

SURBL stands for Spam URI RBL or Spam Uniform Resource Identifier Real-Time blocklist, and it contains information about websites included in the body of spam messages. SURBL.org was the first to offer the service in 2004.

SURBL is complementary to DNSBL and is used to check if websites mentioned in spam emails are included in any active blocklists. If blocklisted websites are found in such emails, the emails themselves are flagged, and IPs associated with them may be blocklisted too. 

Abuse, phishing, malware and cracked websites are included in SURBLs, but they do not involve senders. All in all, both DNSBL and SURBL systems provide invaluable data that assist spam filters and help fight abuse online. 

How to check if your IP address is blocklisted

There are numerous virtual tools dedicated to providing anti-spam and virtual reputation blocklists. A few of them include SpamCop, Barracuda, SORBS and Spamhaus. These services produce their own blocklists that help determine the overall reputation of an IP or domain. 

MxToolbox, MultiRBL, What Is My IP Address and other analogous services online provide free IP blocklist check tools. Using these IP blocklist check tools, you can access data regarding your IP or domain status.

If you discover that your IP or domain was blocklisted with the help of these tools, your next step is to figure out how to remove/delist the IP address from the blocklist.

IP Blacklist Check performed using MxToolbox.
IP Blacklist Check performed using MxToolbox

How to remove your IP address from the blocklist and prevent future blocklisting

Is your IP blocked? If you find your IP blocklisted after performing an IP blocklist check and you wish to continue providing services, you need to get your resource delisted/removed from that specific blocklist.

How to delist IP addresses from blocklists

Needless to say, proving that your IPs are not associated with the malicious or suspicious activity is a process that may require time, effort, and patience. Fortunately, some blocklists delist IPs automatically once the IP blocklisting issues are resolved. In this situation, once you run a blocklist check again, your IP address should appear with a clean record. 

Here are the steps you may need to perform to get your IP address delisted from a blocklist.

  1. Find the blocklist that blocklisted the IP address.
  2. Contact the blocklist to identify the reason your IP was blocklisted.
  3. Resolve the issue.
  4. Request removal from the blocklist.

Every blocklist has unique requirements and procedures when it comes to delisting IPs. Essentially, if your IP address is not delisted automatically, you need to resolve the violation and then contact each blocklist that flagged you individually to request removal. 

Removal Request form the Barracuda Reputation Block List (BRBL).
Removal Request form the Barracuda Reputation Block List (BRBL)

How to keep IP addresses clean

Without a doubt, preventing IP addresses from landing in blocklists is easier than getting them delisted. If you want to save time, maintain email deliverability and healthy email traffic, decrease client complaint rate and prevent blocklisting in the future, there are a few things to keep in mind: 

  • Exercise transparent email marketing practices
  • Curate your email content carefully so that your recipients wouldn’t flag it
  • Always include an unsubscribe option in all emails
  • Do not purchase an email list or send spam on purpose
  • Set sent email limits and stay consistent with them
  • Use an SPF record for anti-spoofing purposes

Essentially, if you use your IP address in a transparent way, your chances of avoiding a blocklist are much higher. Of course, IP addresses associated with mail servers and email marketing companies are not the only ones that might be affected. 

Regardless of what services you provide, remember to always keep your systems secure, perform regular anti-malware checks, avoid illegal activities, do not pay for email lists and do not ignore email notifications warning you about suspicious behavior. The quicker you act, the quicker you are likely to resolve the problem. 

While it may not be the end of the world if you end up on one blocklist, it might only be a matter of time before other blocklists identify the same violation. 

IPXO and IP blocklists

A strong reputation is essential for any legitimate business, and at IPXO, we understand the need to maintain a spotless IP reputation. This is why advanced anti-abuse practices support our automated IP monetization and leasing services.

First and foremost, we screen all clients who wish to bring IP addresses to the IPXO Marketplace. This helps us identify and handle IP reputation issues even before IPs are added to the Marketplace. During the process, we also vet the clients themselves to ensure they are legitimate and trustworthy. 

Even after IPs are leased, we continue to monitor them and perform daily health checkups. IPXO uses different blocklists to make sure that no IP reputation issues are overlooked. We combine automation and the expertise of professional anti-abuse and security teams to guarantee thorough IP monitoring 24/7.

About the author

Gustavas Davidavičius

Senior Abuse Prevention Specialist

Gustavas is a Senior Abuse Prevention Specialist at IPXO. He specializes in the abuse desk policies, as well as IP reputation upkeep on the IPXO’s IP leasing and monetization platform.
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