Blog , 10 Mar 2021

The Essential Guide to Blacklisting

Tobias Knecht,

Founder and CEO, Abusix

A blacklist is a company’s worst nightmare. Landing on a blacklist means your email network is suspected of abuse and blocked as a result. That means your outbound emails will get funnelled to the dreaded spam folder, or if you’re an ISP, your customers are unable to do business. It can be tough to get off a blacklist and, while you scramble to resolve the problem, your business and reputation will suffer. Nobody wants that.

To help you avoid them, we created this helpful guide to blacklisting. It explores what they are, how they work, why your IP or domain might be on one, and what you can do to prevent being blacklisted in the future. Prevention is always better than the cure!

What Is a Blacklist?

A blacklist, also known as a blocklist, is a list that identifies IP addresses and/or domains that have been seen sending spam emails or have done something contrary to a provider’s terms of service.

It’s rare to appear on a blocklist for no reason, it might be because of a compromised system, stolen user credentials, or because you sent a newsletter to a purchased prospect list. 

How Does a Blacklist Work?

Either the sending or receiving email server checks the sending domain and/or IP against the blacklist and then restricts or allows the email to be sent or received. They’re typically compared on the receiving end only, and the email is bounced back automatically to the sender.Sometimes, the emails are let through and end up in a Spam folder, but it depends on the blacklist and how the email spam filters are set up. 

Who Owns a Blacklist?

Blacklists/Blocklists are created and maintained by security companies such as Abusix. Each company will have its own method and policies for developing its lists. Typically they are based on current threats, security best practices, and observations. 

Note: If you want to understand how Abusix’s lists are being built, check out this blog post series here.

Real-World Scenarios That’ll Get You Blacklisted

  1. If you use shared hosting for email, then it is possible that a compromise of another account or website on the same host might cause the hosted IP to become listed.
  2. Infected PCs or devices inside a network can cause the external IP address of your network to be listed, which will cause issues if email is sent from the same address.
  3. If you launch an email marketing campaign that far exceeds your historical email volume. 
  4. If you use any cloud services and your vendor re-uses IP addresses, you could be assigned on blacklisted when it belonged to another customer. 
  5. If your IP or domain receives a large number of spam complaints, you’ll most likely be blacklisted.
  6. If you have compromised user accounts or web servers, you are at high risk of being blacklisted. 
  7. If your site has vulnerable or abusable web forms, these can be used to send spam by proxy or to compromise your server with malicious code or pages. 
  8. If your email newsletter list has a high hard bounce rate. Hard bounces are signs to email servers, ISPs, and email service providers (ESPs) that recipients might not have opted in to your emails.

How Do I Know If My IP or Domain Is on a Blacklist?

If you find messages that you are sending are being returned (bounced) or your messages are ending up in the recipients “spam” folder, then you might have been listed on a blocklist.

If your messages are being returned, the “bounce” message that you receive back usually contains all the necessary diagnostic information required to determine the problem, so read it carefully.

There are also a number of online lookups tools that you can use to check to see if you’re on a blocklist.

The Abusix Lookup tool for example checks your domain, IP, or email address and will tell you if the domain, IP, or email address appears on any of our blacklists. 

Click any of the results to find out more information about the blacklist. You’ll get more information on why your IP or domain has been listed in some cases will give you further information to help you fix the issue.  You can always use the live chat at the bottom of the page to talk with the support team if you require any more help. 

Once the issue has been fixed you can request a delist. You’ll then get forwarded to app.abusix.com, where you’ll need to create a free account in order to delist/remove your IP or domain.

Screenshot of lookup page "lookup.abusix.com"

To do a wider search of lots of different blocklists try MXToolbox and MultiRBL. Their approach works quite similar.

Note: The maintainer of a blocklist doesn’t block you, the receiver who uses the list does.

Maintainers of blocklists are simply providing a data service that can be implemented and used by an endless number of email receivers.

These receivers decide if and how much trust he’ll put into the available data. One receiver might completely block every IP on the list while another one might only use it as an additional indicator.

The blocklist maintainer provides data that is trusted by its customers for the purpose of their own email security, so you should never blame the maintainer, instead, you should always check your own stuff first.

How to Avoid Blacklists

Here are seven ways to avoid blacklists for your website, domain, IP, or email address.

1. Track Your Email Stats

If you’re for example sending newsletters, your marketing team is probably already tracking email opens and clicks on all their email campaigns, but they should tell your security or IT team if they notice anything dramatic. A big drop in these numbers can indicate a problem that you’ll want to check immediately. 

Also, your organisation should collect the information about any email or domain delivery issues. Many security teams don’t dive any deeper into the actual issue. However, if you don’t uncover why it’s happening, you’ll be forced to address it every time it occurs. This is a bad use of your team’s time and efforts, so have them collect and analyze the issue to uncover the root cause. 

2. Communicate Spam Policies to Customers

You should always have your customer’s permission to email them, but with GDPR in Europe and other anti-spam laws popping up globally, this is more vital than ever. Use double opt-ins for email newsletters to make sure you have their explicit permission. Publish your email and privacy policies on your website and ensure it’s easily found so anyone can take a look as needed. 

In case your domain, IP, or email is blacklisted, you’ll want to reach out to the blacklist owner,  email service provider, or website host to find out what’s going on. Some will be able to provide additional information about your case so you can fix it right away. Either way, communicate with them to let them know you’re going to fix the problem right away. 

If you’re the ISP, ESP, or related web provider to customers who send large amounts of emails, you’ll want to ensure they’re following email best practices too. Create and publish clear spam policies that indicate how you’re tracking them and what happens if they get flagged by the blacklist. Check out the Messaging, Malware and Mobile Anti-Abuse Working Group (M3AAWG) for more information on best practices for electronic communications that reduce messaging abuse and will help you and your customers stay safe.

Also, read the 8 ways on  how ESPs and DNSBL provider can collaborate better.

3. Verify New Customers

By confirming that new customers are really new customers, you’ll keep a good number of hackers off your systems. Hackers often use fake names, addresses, and stolen credit cards to get access to your online services.

They quickly take over your network, systems, or infrastructure before your billing provider tells you about the chargeback. Not only does this give you a ton of extra-billing overhead and costs, but it also can get you put on a blacklist pretty quickly. A customer verification process will prevent them from subscribing and give you a cleaner network and profile online.

4. Personalise Customer Emails

Another easy way to avoid blacklists is by personalizing the emails your company or website sends to people. A generic mass email feels so impersonal that people often complain about them. Too many of them doing that could result in you getting on a blacklist. 

However, a personalised email feels like a conversation, and people are less likely to report them as spam. Consider segmenting your list based on metrics that make sense for your business, such as geography, demographics, and interests. Also, consider writing better subject lines to your emails. Not only do personalised subjects have a 26% higher open rate, but they also seem less spammy and can reduce complaints.

5. Protect Your Systems with the Right Technology

Smart deployment of technology solutions can help your website and network stay safe from hacks and online attacks. They’ve got the added benefit of keeping your site, network, IPs, and email addresses safe from blacklists. Most of these help you monitor your system proactively so you can detect and address problems before they get the attention of blacklists. 

5a. Use an Outbound Email Filter

A strong outbound email filter will help you better understand the emails that pass through your network. It will flag spam emails using the very blacklists you’re trying to avoid while helping the blacklists and filters understand what’s safe and what’s not. A sudden increase in spam flagging by the filter could indicate a larger problem with your network too, giving you a chance to fix it faster. 

5b. Secure Your Network with a Firewall

A firewall can help you monitor network traffic and protect your network when a system is compromised. You can flag the appropriate connections while you investigate, thereby preventing your network from being used by hackers and getting you put on a blacklist. 

5c. Encrypt Access to Your Network

Encryption technology can protect your website and network’s entry points so hackers can’t use them illegally. Here are a few options to protect your network from being used by malicious actors for illegal activities, which can get you on blacklists without knowing it.

  • SSH keys protect servers and other important infrastructure from brute force cracking attempts to prevent access to these systems.
  • Public key infrastructure (PKI) security can control who has access to your website and network, only allowing authorized people, devices, websites, and applications through. 
  • SSL/TLS encryption on your data transmission points protects your data as it travels in your network. 

5d. Segment Your Network

This segmenting tip will prevent your entire network from being placed on a blacklist. When you segment the network on different systems, IPs, and infrastructure, you ensure that only one part of the network could be blacklisted at a time. Showing up on a blacklist will no longer cripple your entire network or business. For example, segmenting your network into corporate-use, customer-facing, and customer-owned networks. 

5e. Add an Intrusion Detection System (IDS)

Another layer of security you can add is an intrusion detection system (IDS). An IDS is a software solution that monitors network traffic for suspicious activity and warns you when such activity is discovered. They monitor your systems for signatures of known attacks and deviations from normal activity (another signal of something bad going on.) 

Adding an IDS to your network security tech stack can help you detect things as they happen, so you can fix them before you get put on a blacklist. 

6. Stay Updated on Blacklists

The final tip on avoiding getting on blacklists is to stay updated on the latest information about them. Review and analyze all the reports you have on network activity, read security publications, and do anything you can to stay ahead of them. The more your organisation knows about blacklists and how they can negatively affect your business, the better chance you’ll have to stay off them. 


Blacklists are a useful tool for the online world today. They protect your business and infrastructure from unauthorised use. But that same protection can be turned into a headache for your business if you’re unknowingly put on one. Depending on what’s been blacklisted, it could even affect your entire business and prevent you from sending emails or being found through search engines. 

Being proactive and understanding how your network works normally can help you minimise the risks of any part of it being blacklisted.

Whether it’s by using a software solution that monitors and alerts you to anomalies or by changing your employee processes and procedures, every action can help.

Blacklists aren’t going away and will likely increase as technology evolves, they’ll probably evolve in parallel. 

If you are running your own mail servers and want to protect your customers from receiving spam, malware, or phishing attacks, you can go ahead and start a free trial of Abusix Mail Intelligence as the first layer of defense to protect your mail servers and customers from malicious email-related attacks.

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