Email Blacklists Explained: Causes, Solutions, Prevention

Picture this – it’s nine AM, and you open your laptop just to be greeted with a flood of bounced emails. Well, that’s what happened to me the first time I ended up on an email blacklist.

Today, I’ll take you with me on a journey of understanding what blacklists are and how they work. I’ll also show you what I had to do to get unblacklisted and how I prevent getting booted off the email grid from happening again.

Let’s dive right into it!

What is an email blacklist?

An email blacklist is a database of IP addresses or domains suppressed for malicious behavior. It also goes under the technical names of the Domain Name System-based Blackhole List (DNSBL) and Real-time Blackhole Lists (RBL). 

Major email service providers (ESPs) such as Gmail, Outlook, and Yahoo all have their own email blacklists that are regularly updated and shared amongst each other as well as with internet service providers (ISPs) and anti-spam vendors.

Types of email blacklists

There are about 5 types of email blacklists, but things can get a bit confusing, so stick with me while I explain.

First, you have domain-based and IP-based blacklists, where the main difference is that domain-based blacklists focus on the sending domains, while IP-based blacklists focus on the email server IP addresses. 

Then there are public and private blacklists, where one is accessible to the general public, and the other is reserved for the use of major mailbox providers and requires a license to be accessed.

Finally, enterprise spam firewalls are a type of blacklist provided and maintained by independent organizations such as Spamhaus, Spamcop, Invaluement, and Barracuda and are used as a security mechanism by IT departments.

Among the popular blacklists, you will find a mix of all the mentioned types, with the 10 most popular lists being:

• Spamhaus Block List (SBL)
• XBL Exploits Block List
• DBL Domain Block List
• PBL Policy Block List
• Composite Blocking List (CBL)
• Spamcop
• Invaluement
• Barracuda
• Passive Spam Block List (PSBL)
• SenderScore

How does an email blacklist work?

Email blacklist filters work by helping ISPs decide if an email should be accepted or rejected. And they’re part of a complex email filtering process that looks something like this. 

After an email has been sent, it reaches an ISP. The ISP then checks the email’s IP addresses, domains, or hashes against various blacklists to determine if they are listed. Additionally, as a second layer of defense, spam filters analyze the message to ensure it complies with ISP policies and does not contain malicious or unwanted content.

If an email passes both checks, it is delivered to the recipient’s inbox.

If it does not pass, the email may either be completely rejected or accepted with a tag allowing further filtering by the recipient’s email system.

For a visual explanation of how and when a blacklist is used, check out the graphic below created by Spamhaus:

What are the reasons to blacklist an email address?

Poor email list hygiene 

On an email contact list with poor hygiene, you’ll find lots of inactive addresses as well as a few spam traps. Sending emails to inactive subscribers will demonstrate potential neglect of honoring unsubscribe requests while emailing spam traps might raise suspicions that you bought your email list or don’t implement opt-in solutions.

A lot of spam reports

Spam reports are something every email sender will have to deal with. However, if the number of spam reports against you is significant, ISPs have no choice but to blacklist you. And although an increase in spam reports could happen for a range of reasons, it usually stems from general uninterest from your audience or lack of proper opt-ins.

Poor or spammy email content

Content with an overly promotional tone, spam words, or content that lacks relevance can often trigger not just ISP filtering mechanisms but also recipients to flag you as a spammer. So, before using keywords such as “money-back guarantee”, all caps, and excessive exclamation marks or skipping on creating engaging and valuable content, do keep in that it could be a fast track to blacklisting.

Hacked or compromised email account

In the case of hacking, your account will be used to send large amounts of spam messages at a time. In less serious cases, your email could get spoofed, and your IP address could be used to send fraudulent messages. Both of these activities, when registered by ISPs, will ruin your sender reputation and cause immediate blacklisting.

Spikes in the volume of outbound emails

Inconsistency in outbound email volume is almost always a characteristic of malicious email senders. So, if you have a sudden and sharp spike in your daily sending volume, ISPs might consider it suspicious activity and blacklist your IP/domain in turn.

Malware infection

Sending emails from a malware-infected system is guaranteed to prevent inbox placement. So, if your system is compromised by a virus, spambot, Trojan, or any other type of malware, it will not only harm your email deliverability but will also likely result in your immediate placement on a blacklist.

How to check if your email domain is blacklisted

Use spam report information

Spam reports can be a goldmine of insights into if and why your emails might be landing in the junk folder, and depending on the tool you are using to generate them, you might even get a dedicated blacklist report for your IP or domain. 

Mailtrap, for example, comes with a built-in spam checker intended to prevent your emails from going to spam and improve deliverability. 

For each email sent, the spam checker will generate a spam report using Apache SpamAssassin, which includes a general score and a detailed table with spam test points and their descriptions, as well as a blacklist report.  

Through the blacklist report, you can gain insight into whether your IP or domain has been listed on any of the commonly used blacklists, see resources that have been queried, and check your current blacklist status. Also, in the case of your IP or domain being blacklisted, Mailtrap’s spam checker will provide you with the name of the blacklist and a hyperlink to its website so you can quickly check the rules for delisting and follow the instructions.

Use blacklist checker tools and blacklist monitoring tools

As blacklists can be divided into IP-based and domain-based categories, the blacklist checkers available to you also come in two different forms. Still, regardless if you need an email IP blacklist checker or an email domain blacklist checker, make sure your pick is a publicly available tool, as they are quick and easy to use.

Here are the most popular ones to check out:

• MX Toolbox
• DNS Stuff
• DNSBL
• MultiRBL 
• Spamhaus

Along with blacklist check tools, also consider using a blacklist monitoring tool such as HetrixTools and Debouncer that will continuously check your IP or domain and notify you if you’ve been added to or removed from any list.

Monitor email deliverability rates

Even if you haven’t directly noticed a blacklist issue yet, dips in your deliverability rate could be a sign that your messages are getting blocked or sent to spam. To catch these red flags early, regularly analyze your email campaign reports for any patterns or sudden drops in engagement. 

Listen to feedback from email service providers

Modern email service providers come with features for flagging sudden spikes in spam complaints, high bounce rates, and other deliverability issues. So, one of the simplest ways to detect a possible blacklist issue is by paying attention to direct notifications or warnings from your email platform, like feedback loops, for example. These alerts often come with basic insights or error messages that point you toward the root of the issue, such as your domain or IP address being blocked.

How to get off an email blacklist

If you ended up in the same situation as me and found yourself on one or more blacklists, not all hope is lost, and you can still clean up your email sender reputation. 

Here’s how I did it!

Step 1 – Find the cause 

Once you suspect that you’ve been blacklisted or a simple email blacklist lookup confirms it, your first action should be pinpointing why you ended up on a blacklist in the first place. 

In my case, I went straight to my recent campaign’s email deliverability, engagement, and spam reports and checked for any notifications or warnings I might have missed that could give me a clue.

Step 2 – Fix the issue 

With the problem that caused the blacklisting identified, it’s time to do the necessary fixes. As for me, the issue came from an outdated email list; I proceeded to clean it by removing inactive and invalid addresses. I also ran a permission pass campaign to confirm with the rest of my email list that they are still interested in receiving emails from me.

Depending on what the issue is for you, you might be implementing proper authentication, rethinking your email design and copy, or doing some other type of task.

Step 3 – Request delisting 

The third and maybe the most important step is reaching out to the blacklisting entities and submitting a request for delisting. Of course, I only did this after implementing corrective measures and collecting proof of doing so. 

The delisting request will often entail you filling out a form in which you explain what issues were addressed and show commitment to following best email-sending practices moving forward. But do keep in mind that delisting might not be instant and that for some blacklists, you might even have to complete additional steps before removal is initiated.

If all fails, start from scratch

Although I was lucky enough to be able to fix my issue (and you probably will, too), there is still a chance that a sender could stay blacklisted forever. In that case, starting over with a new domain or IP is the only solution. On top of that, using proactive measures is also a must. Otherwise, you will again get quickly put back on a blacklist.

How to avoid getting blacklisted

Regardless if you’re like me and have been blacklisted before or you’ve never been in email sender timeout, we have a common goal – staying off of blacklists. 

To achieve this goal, I created a list of best practices, which include:

Run spam tests

Spam testing is a practice I implement not just for figuring out if I’ve been blacklisted but also for preventing blacklisting from happening in general.

Of course, in both cases, the spam checker I go for is the one from Mailtrap, as it enables me to:

• run in-depth email spam tests and get a detailed list of spam points and HTML spam rules they go against
• inspect the email subject line, email body, and attachments, and remove any malicious or broken links
• and check if my sending IP address or email domain has been mentioned on any major public blacklists

Using this spam checker, I test all my emails including newsletters, email notifications, cold emails for outreach, or email marketing campaigns. And since all the emails I send to Mailtrap get captured by a fake SMTP server, my testing never causes any actual bounces. 

Pretty convenient, right?

Implement proper email authentication

When it comes to the technical aspect of preventing blacklisting, with every email provider I use, I always make sure to configure SPF, DKIM, and DMARC for my domain.

SPF is there to check that my emails are sent from authorized mail servers, while DKIM adds a digital signature to verify authenticity. Finally, DMARC ties everything together, letting me decide how to handle any suspicious emails. 

Maintain your email lists

Considering that poor email list maintenance is what led to me getting blacklisted, I now take multiple precautions to ensure I am only sending to real accounts and people who are genuinely interested in my emails.

Never buy email lists

First and foremost, I steer completely clear of using purchased email lists. These lists are often outdated, full of invalid addresses, or even worse – spam traps designed to catch bad senders. Building your list organically may take more time, but it’s the only way to ensure your emails reach real, engaged recipients.

Regularly clean your lists

Over time, even the best email lists collect bad data. That’s why regular cleaning is a must and should include:

1. Removing bounced emails: Hard bounces should be removed immediately to avoid harming your sender reputation while soft bounces should be monitored closely, and if they keep repeating for the same email address, consider removing it from your list as well.
2. Analyzing engagement: Pay attention to who’s opening your emails and who isn’t. If someone hasn’t engaged with your campaigns in months, send them one re-engagement email, and if that fails, remove them from your list.
3. Honoring unsubscribe requests: Always respect requests to opt out of your emails. Not only is this the right thing to do, but it’s also required by laws and regulations.

Segment your lists

The email addresses on my list are segmented and grouped based on factors like location, purchase history, or interests. By doing this, I send more personalized and relevant emails, increasing engagement and reducing the chances of ending up in the spam folder. 

Send only good content

To increase my chances of landing in inboxes, I focus on clear messaging that resonates with my audience and steer clear of deceptive subject lines, trigger words like “free” or “cash bonus”, excessive punctuation, and all caps. Along with that, I find that personalization is key—whether it’s as simple as addressing recipients by name or tailoring offers to their preferences .I’ve also learned to maintain consistent email-sending frequency, avoiding sudden spikes that might irritate my audience or trigger delivery issues.

Build trust with subscribers

Building trust with subscribers starts with gaining explicit consent through transparent opt-in forms and honoring their preferences. So, in every email I send, I include a clear unsubscribe link to make opting out easy. For ensuring only real, interested people join my list, double opt-ins are my go-to.

Remind subscribers to move your email to their inbox

When an email address or domain gets blacklisted, it often means the messages sent from it will be automatically routed to spam. To prevent this, in the past, I’ve encouraged my subscribers to “whitelist” my emails, or in other words, move them into their inboxes or add me to their safe sender list. 

Wrapping things up

In this post, we covered everything you need to know about email blacklisting – from what causes it in the first place to what fixes should be made to repair the damage done. So, if you’ve never been blacklisted before, use the information from this article to lower your chances of it ever happening. On the other hand, if you currently find yourself on a blacklist, follow the steps I outlined to improve your situation and make ISPs, as well as recipients, trust you again. 

If I’ve successfully navigated the rough email blacklist waters, you can too!