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Everything You Need to Know About Email Blacklists & How to Avoid Them

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Whitney Blankenship
Content Marketing Manager
Reading Time: 8 minutes

Did you know that 82% of B2B companies use email-based marketing campaigns? It’s true, and for very good reason: Email campaigns have a massive ROI. A well-done email marketing campaign can boost ROI by as much as 4400%—that’s almost $44 for every $1 spent!

Anyone who has run an email campaign has probably found themselves on the backside of an email blacklist. These pesky firewalls can range from being a minor annoyance to a complete catastrophe that derails your email deliverability goals and plunges you into a major, time-consuming clean-up. In fact, it is estimated that one out of every five emails you send never reaches the inbox of your recipients,  likely due to an email blacklist and other spam filters.

Email deliverability is the no. 1 metric for email marketers. So before you dig into the data to analyze open rates, you need to understand the basics of email blacklists, check whether you are on one, and know how to avoid them.

What Is an Email Blacklist?

Approximately 80% of emails sent every day are considered spam, making email blacklisting a necessity for maintaining a manageable email structure.

Blacklisting is the practice of identifying IP addresses associated with spam content, and then blocking content from those addresses. The point of an email blacklist is to prevent unwanted spam content, sent by untrustworthy sources, from cluttering inboxes. You can think of blacklisting like blocking someone on social media: they can no longer comment on your profile or send you private messages. Relating back to email terms, this blockage harms your deliverability. 

The IP data of a suspicious account is added to a grand list in email servers —a Domain Name System (DNS) blacklist—which then blocks emails generated by the blacklisted IP addresses. This diverts unwanted messages away from the recipient’s inbox and into the spam folder. Further, if you have a Gmail account, the spam folder usually leverages a type of email blacklisting that redirects spam messages to another folder.


How Do Email Blacklists Work?

Whenever you mail a message electronically, it contains a string of digits called the IP address. The IP address uniquely specifies the server that the email came from, kind of like how a letter in the mail has a return address. You can trace the location of an email by referencing the IP address.

When inbound mail is received by an ISP, it automatically checks the IP address against an email blacklist or, in technical terms as previously mentioned, the DNS blacklist. If the incoming IP address is on the list, the email is discarded. If it is not on the list, it is checked against some more spam filters before reaching the intended inbox. The nature of these other spam filters is up to the ISP in question. In that sense, a blacklist is just one tool that determines whether email deliverability is successful.

ISPs identify IP addresses associated with sending spam content or emails to addresses that are spam traps. On one hand, IP addresses are identified based on their history of sending good or bad emails. Repeated offenses easily attract the attention of spam blacklist providers. 

If the IP address usually sends quality mail, it is cleared for delivery. Take note, however, that a good history doesn’t shield you from the spam list. Any message containing spam traps will push you to any number of email blocked lists in seconds.

There are 3 main types of email blacklists you can 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. More on this later. 

What Does Being Blacklisted Mean?

Being blacklisted means that your emails will not reach your intended recipients. This can wreak havoc on your marketing efforts, especially if a large portion of it is email based. Even worse, 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.

Sometimes your IP address is not blacklisted, but the specific originating domain is flagged as the culprit. Email domain blacklists are functionally the same; they just root out the email at a different level within the security flowchart.

How Do You End Up on a Blacklist?

Something as simple as an email list with bad addresses or increased spam complaints can earn you a ticket to an email blocking list. Here are a few common things that can harm you as a 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. Additionally, many ISPs use harvested emails as pure spam traps. Attempting to send many emails to the inboxes of these accounts reflects poorly on your company’s email sourcing practices.
  • Hackers: Hackers may have gotten access to your account to send spam messages. Alternatively, hackers could be spoofing your email address, which means they are piggybacking off your IP address to send fraudulent emails. If you get a lot of error messages for emails you did not send, then someone might be spoofing your account.
  • High volume of emails: Spammers send the most emails in the world (remember that 80% figure referenced earlier?), so if an IP address has a much higher email delivery volume than other IP addresses, it is likely a sign of a spammer. In general, the higher the volume of emails originating from your IP address, the more likely it is to end up on a blacklist.
  • Increased email volume: If an ISP notices a sudden spike in email traffic coming from your IP address, it can take that as a sign of spam content. Legitimate organizations usually build their email list slowly over an extended time and sudden spikes look very suspicious.
  • Spam complaints: Every company will receive some complaints about their email marketing. In itself, that’s not necessarily a bad thing. However, if the number of complaints reaches beyond an acceptable threshold, an ISP might put you on the blacklist.
  • 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. While these guidelines are more applicable to B2B marketing than ecommerce marketing, keeping your copy clean can help you avoid potential negative ramifications.
  • Failing to manage your email list: If you do not regularly monitor your email list, and neglect to honor unsubscribe requests, then you are much more likely to be ordered to an email blacklist.

ESPs rely on the ability to send massive email quantities, so they usually attempt to manage ISP blocks directly. If your email is the reason an ISP created a block, though, the email provider may have some questions for you regarding your email and send practices.

How Many Emails Can I Send Without Getting Blacklisted?

It’s not just the raw amount of mail electronically delivered that can get you blacklisted.

Email frequency and a sudden increase in volume can affect you.

Big companies send thousands of emails every day, but they are neither blacklisted nor directed to spam because they have gradually grown their mailing list over time and exhibit good email etiquette.


If you begin sending a ton of emails compared to a baseline of relative inactivity, then there is a strong likelihood that ISPs will flag your IP address for suspicious activity. Email volumes need to grow organically in order to dodge blacklist and spam filters.

How can you warm up your email marketing sender reputation?
Learn here!

How Can I Check If My Email Has Been Blacklisted?

Has a low email open rate ever triggered the question, “Are we blacklisted?” The fastest and easiest way to check is by using a publically available tool. Common DNS lookup databases include:

These free tools will check your IP address against more than 120 publicly available blacklists.

You also should keep an eye on email campaign stats, including open and click rates. More importantly, persistently check domain opens. If you notice a sharp drop-off of domain opens, it is a good indicator that you may have been blacklisted from an ISP.

Sometimes, however, you won’t be able to detect if you are on any email block lists. That is why it is very important to take active steps to prevent yourself from being blacklisted by using good email sourcing practices and routinely monitoring your email campaigns.

What Should I Do if I’ve Been Blacklisted?

Sometimes you can do everything perfect—maintain clean lists, be diligent in creating messages aligned with high deliverability best practices—and still find yourself assigned to an email blacklist.

Being on a blacklist is not the end of the world. Nearly 15% of marketers said that their companies’ email addresses had been blacklisted at least once in the past 12 months. Also, blacklisting does not appear to put a huge dent in marketing efforts. Marketers who described their campaign as successful were just as likely to be blacklisted as others.

If the data shows you’re on a blacklist or two, don’t panic. Fortunately, nearly every major blacklisting company offers ways to appeal your status and have your IP address removed from the list. In general, the process is quite simple. Just contact the company and proceed with the actions they prescribe to clean your email profile.

And sometimes, a company might banish 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 usually fix the problem as well. The process typically involves them contacting you to remove the IP blocks. 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

There are a few practices you should regularly adhere to if you want to avoid a blacklist sentence. 

Keep your email list curated

Email lists with bad addresses are the predominant culprit for being put on an email blacklist. Never buy a list, as the addresses are likely illegitimate. Worst case, some addresses are spam traps, which almost guarantees your appearance on a list of spam email domains. 

Here are some ways to clean your list: 

  • Remove any 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 open rates. 
  • Always honor client requests for removal from your email subscriber list.
Never buy an email list

Use double opt-in for email subscribers

A double opt-in is a two-step email verification process. In the first step, 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 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.

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.

Avoiding Email Blacklist: Wrap Up

Unfortunately, nearly every company will be placed on one or more email block lists 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. The key thing to remember is that a strong email-based marketing approach hinges on good email etiquette and a clean list, which drastically reduces the odds of being added to a mail server blacklist.

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Whitney Blankenship

Sr. Content Marketing Manager for Omnisend. When not writing awesome content, Whitney is reading up on the latest in digital marketing, ecommerce, and social media trends. Obsessed with pop culture, art, and metal. Powered by coffee. Fastest Googler in the West.