DNS Blacklist: What It Is and How It Works?

What is a DNSBL?

A DNSBL or Domain Name System Blacklist is a part of a system that helps to fight email spammers. There are a lot of such lists in existence, each of them containing a list of IP addresses known to send spam. 

DNS blacklists are used by email services and website admins to effectively keep spam out of their clients’ inboxes. Think of such lists as bouncers, but instead of having a list of people to let in, they contain lists of people to keep out. 

Despite its name, the term “list” is an oversimplification. These “blacklists” are not simply lists but rather security mechanisms that utilize various methods of checking whether or not a specific address or domain is listed, gets listed, or is removed from the list. These lists can also sometimes be referred to as “blocklists”. 

Some of the most common DNSBLs include:

• Composite Blocking List (CBL)
• Spamhaus Block List (SBL)
• Spamcop
• Passive Spam Block List (PSBL)

And many more.

How are blacklists created?

There are a few different ways that blacklists can be created depending on the provider that maintains them.

1. Blacklists are created through spam traps and email honey pots. These are email accounts monitored by blacklist operators. If a spammer sends an email to a spam trap, their DNS can get listed and blocked. 
2. Blacklist services monitor and list down addresses of zombie computers (virus-infected computers used for malicious purposes) or bots that are known to send spam. These zombie computers can also be originators of malware and other harmful items.
3. Blacklist services take note of internet service providers (ISPs) that have hosted or are currently hosting domains that send out spam mail on their servers.

How does one get on a blacklist?

You could potentially be added to a DNS blocklist because:

• Your domain sends emails marked as spam.
• Your domain contains viruses and malware.
• Your domain has been hacked or is suspected of hacking activity.
• Your ISP is associated with spam activity.
• Your country is blacklisted.
• Your server has malware or lacks proper configuration.

Consequences of being added to a DNS blacklist

Getting added to a DNS blacklist can be extremely harmful to those who are just looking to market their brand or business by doing email marketing. If you are on the list, all emails from your domain will automatically be blocked regardless of their content.

And it’s not just your emails. These lists may also block potential clients from seeing your domain altogether, thus greatly hindering your online presence and ability to promote your brand and company.

This is not the end of the world, though, as you can still get out of these dreaded “lists”, more on this later in the article.

Depending on the email service provider, an email sent from a blacklisted domain may either be blocked entirely or redirected away from the primary inbox and into a designated spam folder.

How to find out if you are on the blacklist?

If you are the owner of the service, you are likely to get a notification from a blacklist service.

Oftentimes emails bouncing a lot more or the sender reputation suddenly plummeting can be the first signs of getting blacklisted. 

So, if you think you’ve landed on the blacklist (or several blacklists), there are many ways you can go about finding out.

Using tools and services

The best and easiest way to find out whether or not you’re blacklisted is by checking the actual blacklists’ websites. These services are easy to use and provide the most accurate information on your blacklist status.

1. MXToolbox

MXToolbox is one of the most recommended checkers in the business. Plus, it is email-centered, making it especially great for email marketers. It comes with free and paid services.

For free, you can check a single domain per week against 100+ blacklist databases at no cost. For a subscription of $129 to $399, you gain access to their delivery center and monitor email performance for up to five domains and 5,000,000 emails.

Here’s us having checked the selzy.com domain:

2. SpamHaus

If you want a simple, no-nonsense check, you might want to consider SpamHaus. Using this website checker is extremely easy. All you need to do is type in your domain in the search bar and look it up.

While they may not offer automatic monitoring, checking your domain against their blacklist databases is always FREE.

3. DNS Checker

DNS Checker is yet another example of an easy-to-use blacklist database checker. You can check a domain, an email address, or even an IP address against 50 blacklist databases.

This provider doesn’t offer automatic monitoring either. However, their website provides many other services that could be useful for those looking to check their online presence and safety.

4. SiteChecker

SiteChecker also offers its website a blacklist checker for free with the option of searching using domain or IP. It also provides additional information regarding DNSBLs.

This checker doesn’t seem to allow automatic monitoring as it focuses on website SEO support. However, it lets you check for website critical errors after blacklist checking.

Sending a test email

You can also check if you’re on a blacklist by sending a trial email to a domain, site, or email service that you suspect has used a blacklist to block you (for example, Hotmail). Several email services then can alert you that sending has failed and for what reason. 

Others, on the other hand, do not alert you and let you believe that the email has passed with no problem. In this case, use other methods.

Checking email metrics

You can try checking your email metrics if you don’t get any alerts that your emails have failed to send and still suspect that you have been blacklisted. Usually, significantly decreased open rates and increased email bounces indicate that your domain may have been blacklisted.

How to remove your IP from the list

While it may be scary to discover that your domain is blacklisted, it’s not the end of the world. Several blacklist databases often remove you from their list after a certain period of time. And if they don’t, there are several workarounds so that you can get your domain out of their database.

Identify what blacklists you’re on

First things first, you need to establish which services have put you on their lists. To do this, you can check the methods listed in the section above, “How to find out if you are on the blacklist?“

Contact the administrators of blacklist services

Next, once you find out which blacklists are blocking your domain and emails, you need to contact them. As stated, there are several reasons that your domain and emails could have ended up on a blacklist by accident.

Usually, services are more than happy to support you as you fix the issue. Just make sure to ask why your domain was blacklisted and explain your situation as best as possible. You can even take pre-emptive actions (check for spam, malware, and potential suspicious activity) and  fix problems potentially causing your domain to be flagged.

Once they respond, correct the issues (if any) and hopefully reach a point that leads you to be removed from the site’s blacklist.

For example, here’s how a Spamhaus’ notification looks:

They say that an IP was blocked because of an unauthorized mass mailing to unverified email addresses and give recommendations on how to get out of their blacklist. Spamhaus offers to send an email to their team and explain what happened and what actions you’ve taken to solve the problem. After you send this message, the SpamHaus team responds in 24 hours and informs either that the IP address has been removed from the blacklist or that they need additional information.

Run security scan

Many domains are flagged and categorized into blacklists due to malware and viruses on their web servers. With that, it’s best to run a security scan through your domain to ensure that this is not the cause of your listing.

We also suggest you run a security scan after performing any changes and responding to any issues brought up due to the blacklisting.

Top tips on how to reduce the risk of getting onto the DNS blacklist

There’s no need for you to end up on a DNS blacklist at all. All you need to do is keep the following tips in mind.

• Avoid spammy content

Take measures to ensure that the content you have in your emails is as “un-spammy” as possible. That means avoiding high volumes of “spam words” like “buy direct”, “earn money” or “free investment”, using just the right amount of images, and segmenting your lists to ensure that your recipients only obtain information that is relevant to them. 

• Time your campaigns

Sending out campaigns too often can raise several flags and just might mark you as a source of spam. 

Space out your campaigns and allow a reasonable amount of time between campaigns. Typically, campaigns and other promotional mail should be sent out no more than weekly or every two weeks. 

You can even experiment with the best time to generate optimum results. To read more on how to do that, check out our guide on the best time to send an email according to studies (with examples).

• Avoid buying email lists

While this may allow you to garner several emails quickly and with little effort, buying email lists can also put your account at significant risk. Plus, you violate your recipient’s right to consent to receive emails, thus increasing your chances of being marked as spam.

• Personalize your emails

Make sure that your recipients only get campaigns and news that are relevant to them. This increases not only the chances of engagement but also potential sales. You can do this by segregating your email lists and formulating targeted campaigns.

Learn more about this by reading email targeting and how it works.

Conclusion

Finding your domain in a DNS blacklist is the stuff of email marketers’ nightmares. Not only is it extremely easy to get in, but it can also be quite challenging to get out. On the bright side, it’s not impossible to recover from this. All you need to do is to learn how to send mass email properly:

• Make sure all your recipients have consented to receive emails from you.
• Avoid sending a barrage of emails to your subscribers.
• Ensure that your domain and email service providers are secure.
• Keep a close eye on your email metrics for potential signs of being blacklisted.

If you suspect that your domain or IP is blocked by a certain blacklist, make sure to contact the administrators of the blacklist as soon as possible and explanat the situation. So long as you submit to regulations and change whatever is marking you as a potential threat, you shouldn’t have anything to worry about.