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The 101 on email blacklists (in plain English)

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Your email campaign open rate hovers around 18%. Then suddenly one day, it plummets to 5%.

Maybe it was just a blip, you think. Until it happens with the next email…and the next. Now you’re in full panic mode. Suspicions of that feared email marketing dead land—the email blacklist—have reached a crescendo.  

But how do you truly know if that’s the cause? And why would you have been banished to the blacklist?

We answer both questions, as well as how to break free from an email blacklist and steps to avoid one in the first place. 

But first, it’s critical to grasp what a blacklist is and how it affects your email marketing efforts. 

What is an email blacklist and why is it harmful?

The entire point of an email blacklist is to prevent unwanted spam content, sent by untrustworthy sources, from cluttering inboxes. They are absolutely needed: research shows that 47.3% of all emails in 2020 were considered spam.  

If you want the abridged version of how an email blacklist works, you only need to know the next two paragraphs. 

An email blacklist is a record that contains IP addresses or domains flagged by spam filters. Servers then use these lists to identify addresses to block.

Sometimes messages sent from the address in question are directed to a spam folder. Other times they are blocked completely.

Explanation of how email blacklist works

Plainly put, blacklists are a barrier to reaching your subscriber list. 

The nuts and bolts of an email blacklist 

Here’s a more in-depth explanation for those who want to feel like a techie. 

Whenever you mail a message electronically, it contains a string of digits called the IP address. This address uniquely specifies the email originator’s server—kind of how a physically mailed letter has a return address. You can trace the location of the email sender by referencing the IP address.

Technical explanation of email blacklists
Here’s an example from an email I got from Sailthru. When looking at the “code” version of the email, I see the IP address. Following that, we see that it’s been sent from New York City (where Sailthru, part of Campaign Monitor, is based).

Then you have the email blacklists—of which there are hundreds—waiting in the wings to detect anything that appears suspicious. If you want to get real technical, blacklists have a few official names—real-time Blackhole Lists (RBL) or Domain Name System Blacklists (DNSBLs or DNS blacklists). 

So blacklists, as we were saying, are ready to throw up the flag when they notice something funny about an account. We’ll get into the “something” later. 

They grab the IP data and add it to a grand list of fellow bad apples that have been blacklisted.

And as we mentioned, your domain can be the culprit. Email domain blacklists are functionally the same; they just root out the email at a different level within the security flowchart.

What happens when you’re on a blacklist 

Imagine that you have no clue your IP address was put on a blacklist.

So you go about your regular email marketing activities and launch a campaign. We’ll say it’s a sales promotion and John is on the contact list. The email goes to his internet service provider (ISP), which checks your IP address against whatever email blacklists his ISP uses. 

Bingo, there’s a match. Your email is discarded. John either never gets news of your sales promotion or, if he does, it’s delivered to the spam folder that he rarely checks. 

This blockage clearly has a harmful effect on email deliverability. Being identified as a blacklisted email sender can wreak havoc on your marketing efforts, especially if a large portion of it is email based. 

But really…how worried should you be? According to Omnisend Deliverability Manager Serhii Chernenko, it depends on what blacklist you’re on.  

“Some are more influential, respected and impactful, like Spamhaus, Cloudmark and Barracuda. You should be pretty worried if those have you listed, as impact would be significant,” Chernenko says. “Some RBLs will have way less impact on your deliverability or even zero impact, so it is important to monitor the biggest RBLs.”

And it’s absolutely essential that you regularly keep an eye on your email campaign stats, such as click rates. Though the iOS 15 update might skew the metrics, there’s a chance that the open rate could be an indication of something gone awry. Persistently check domain opens for any sharp drop-offs as well.

How do you get blacklisted?

ISPs reserve the right to blacklist any IP address they want, for whatever reason. They are not obligated to hear your deliverability concerns or clean up your non-compliance issues.

There are, however, common things that can harm you as a sender and tag you as a blacklisted email sender:

  • Poor email list hygiene. Failing to regularly clean your list—that is, monitor your data—and therefore send lots of emails to inactive addresses is bad for your sender reputation. That goes for neglecting to honor unsubscribe requests, too. Additionally, many ISPs use harvested emails as pure spam traps, which are email addresses that appear to be real but are not. Sending a lot of emails to the inboxes of these accounts reflects poorly on your company’s email sourcing practices.
  • Hackers. These bad players may have accessed your account to send spam messages. Alternatively, hackers could be spoofing your email address, meaning they’re piggybacking off your IP address to send fraudulent emails. A sign of spoofing is receiving a lot of error messages for emails you didn’t send.
  • High volume of emails. Spammers send the most emails in the world, so if an IP address has a much higher email delivery volume than other IP addresses, it is likely a sign of a spammer. 
  • Increased email volume. Another spam indicator is when an IP address has a sudden spike in sent emails. Think about big legitimate companies that send thousands of emails every day. They gradually grew their mailing list over time and exhibit good email etiquette. Grow your email volumes organically to dodge blacklist and spam filters.
Visualization of email reputation and send volume ratio
  • Spam complaints. Every company will receive complaints about their email marketing from time to time. That’s not necessarily a bad thing. However, complaints that reach beyond an acceptable threshold can be a big problem. 
  • Bad email content. Several ISPs filter emails by keywords found in the body of the text. Words like “money-back guarantee,” text written in all caps, colored fonts, and lots of exclamation marks (!!!!) are all red flag content that can push you to a spam list. This also increases the chance of your subscribers marking your emails as spam. Keeping your copy clean can help you avoid potential negative ramifications. 
example of email that might be understood as spam

How do I confirm if my email has been blacklisted?

If you suspect you’ve made it to a blacklist, how do you confirm or, hopefully, refute it?

There are three main types of email blacklists to check:

  1. Enterprise spam firewalls are normally used by corporate IT departments. Examples include McAfee and Barracuda.
  2. Private blacklists are maintained and circulated by ISPs. Gmail, for example, has its own internal blacklists it uses to monitor and filter emails.
  3. Public blacklists are just that—publicly available, which you can check at no cost. 

We recommend a publicly available tool. They’re fast and easy, and will check your IP address against more than 120 publicly available blacklists. Here are some common tools to check whether you’ve been sent to a blacklist:

Check out the top blacklist tools and why they should be on your radar.   

What should I do if I’ve been blacklisted?

If the data shows you’re on a blacklist or two, don’t panic. Marketers who described their campaign as successful were just as likely to be blacklisted as others.

Fortunately, nearly every major blacklisting company offers ways to appeal your status and have your IP address removed from the list. Some will require filling out a simple form. Others will require an email describing the issue. 

Contact the company and proceed with the actions they prescribe to clean your email profile. 

“Most importantly, you need to address the root cause,” Chernenko says. “A merchant should investigate what exactly led to the listing, take the necessary steps to resolve the situation, and take steps to prevent the same from happening in the future.” 

When trying to delist your IP/domain, provide all possible information you can to the RBL, including what you did to resolve the situation.

Sometimes, a company releases you from the blacklist on its own accord if complaints on your IP address drop off.

If you are using an ESP for email delivery, they can usually fix the problem as well. They’ll typically contact you to remove the IP blocks. However, your account may be suspended until you comply with their demands, such as taking substantial action to clean up your data.

How to avoid a blacklist

Who wants to deal with petitioning for removal from a blacklist? Our best advice is to be proactive so you never have to deal with it. Adhere to the following best practices to stay on the right track.  

Collect emails judiciously

Email lists that contain bad addresses are the predominant culprit for getting sent to an email blacklist.

“The very first and most important rule for any email marketer is don’t ever buy an email list,” Chernenko says. “When buying a list, you don’t know whether or not the email addresses were collected properly.” 

image showing that you can't buy email lists

Worst case, some addresses are spam traps. As mentioned before, they appear real but don’t belong to a real person. Their only purpose is to identify spammers and senders not utilizing proper list hygiene.

Instead, grow your list organically. Collect emails with popups and landing pages on your website. 

Use double opt-in for email subscribers

A double opt-in is a two-step email verification process. Step one, the user signs up for the mailing list on the website. In the next step, an email is sent to the user with a link to confirm their email subscription.

double email opt-in

Double opt-in practices are good for avoiding email fraud, as they ensure that recipients are real people genuinely interested in receiving your marketing emails.

By enabling them, you also avoid complaints, since customers won’t discover they are on a mailing list they did not request to join.

Keep a clean email list 

Collecting emails the right way is one thing. But it doesn’t take care of bad addresses that will inevitably accumulate. Do the following on a regular basis: 

  • Remove bounced email addresses. Make sure to distinguish the hard bounces from the soft bounces, as you don’t want to discard addresses experiencing a temporary hiccup.
  • Examine the performance data. Throw out old addresses exhibiting lackluster achievement, such as weak engagement rates.
  • Honor client requests for removal from your email subscriber list.

Protect your server

It is crucial to check that your server is clean of any malware or bots. These harmful programs hijack your email domain or IP address to send bogus emails, so you can end up on an email blocking list even if you haven’t done anything wrong. These programs can infect your server without your knowledge, so make sure your server security is up to date.

Make it a clean slate

Unfortunately, nearly every company will be placed on one or more email blacklists at some point; it’s just an inevitable part of running a business that values the potential of email-based marketing.

Email blacklisting is not permanent, though, and there are several actions you can take to prevent it from happening and boost the odds that your messages will arrive in a recipient’s inbox. 

If you’re reading this because you’ve run into issues with your own ESP and you’re ready for a clean slate, start Omnisend for free. And make sure to warm up your sender reputation—it will set you on the path toward a blacklist-free existence. 

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Bernard Meyer
Article by
Bernard Meyer

Bernard is the Director of Content at Omnisend, with a passion for good research, helping ecommerce businesses with their marketing automation needs, and beating absolutely everyone in Mario Kart 64.


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