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Email Blacklist: What It Is & How to Avoid It

Email blacklists: Avoidance.

Did you know that 82% of B2B companies use email 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 small annoyance to a complete show stopper, depending on how many you are unfortunate enough to land on. In fact, it is estimated that 1 out of every 5 emails you send never reach your subscribers’ inbox likely due to blacklists and other spam filters.

Given that deliverability is the no. 1 important metric for email marketers, you need to know how to tell when you are on an email blacklist and how to avoid them.

What Is an Email Blacklist?

Blacklisting is the name for the practice of identifying IP addresses associated with spam content and blocking content from those addresses. The point of blacklisting is to prevent unwanted spam content from untrustworthy sources. You can think of blacklisting like blocking someone on social media: they can no longer comment on your profile or send you private messages.

This IP information is compiled into a list in email servers which can then block emails coming from those blacklisted IP addresses. This keeps unwanted spam out of email folders. If you have a Gmail account, the “spam” folder usually uses a type of email blacklisting to redirect spam messages to a separate folder.


Email blacklisting in some form or another is practically a necessity to maintain a manageable email structure. About 80% of emails sent every day are considered spam, so they are pretty useful. The problem is a lot of blacklisting software is automated and sometimes throws the good in with the bad.

Email blacklists can be maintained, updated, and hosted by a company or they can team up with companies that specialize in creating blacklists for different business sectors.

How Do Email Blacklists Work?

Whenever you send a message via email, 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 an email is received by an ISP, it automatically checks the IP address against a blacklist. If the incoming IP address is on the list, the email is discarded. If it is not on the list, it goes through some more spam filters before reaching the intended destination mailbox. 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 an email will be received or not.

ISPs identify IP addresses that are associated with sending spam content or sending emails to email addresses that are spam traps. On one hand, IP addresses are identified based on their history of sending good or bad emails. If the IP address usually sends good emails, it is cleared for delivery. Repeated offenses can land an email address on the blacklist. On the other hand, even if you have a good history but send a campaign to the email list that contains spam traps, you will get on the blacklist in a second.

There are 3 main types of email blacklists:

  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 filter emails.
  3. Public blacklists are publicly available and can be checked for free.

What Does Being Blacklisted Mean?

Being blacklisted means that your emails will not reach your intended target. This can wreak havoc on your marketing efforts, especially if a large portion of your marketing is done through email. Even worse, ISPs reserve the right to blacklist basically any IP address they want for whatever reason, and they don’t really have an obligation to hear you out and fix issues.

Email Domain Blacklist

Sometimes your IP address won’t get blacklisted but the specific domain that the email came from. Email domain blacklists are functionally the same, they just root out the email at a different level in the security flowchart.

How Do You End Up on a Blacklist?

It might seem like you have to meet some very narrow criteria to land yourself or your company’s email address on a blacklist. Actually, though, something as mundane as a bad email list or increased spam complaints can get your put on a list. Here are a few common things that can harm you as a sender with landing you on the blacklist or spam folder.

  • Poor email list hygiene: Sending lots of emails to inactive addresses or inboxes that have been turned into spam traps is bad for your sender reputation. Many ISPs use harvested emails as pure spam traps and sending lots of emails to these accounts reflects poorly on your company’s email sourcing practices.
  • Hackers: Hackers also 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 volume than other IP addresses, it is likely a sign of a spammer. In general, the higher the volume of emails coming 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 goes above an accepted threshold, an ISP might put you on the blacklist.
  • Bad email content: Several ISP filter emails by keywords found in the body of the text. Words like “free”, “money-back guarantee”, CAPS LOCK, colored fonts, and lots of exclamation marks (!!!!) are all red flag content that can get you put on a blacklist.
  • Failing to manage email list: if your email list is unmanaged and you frequently do not honor the request of people to unsubscribe, then you are much more likely to get put on an email blacklist.

ESPs rely on being able to send lots of emails so they usually try to manage ISP blocks themselves. If your email is the culprit though, they may have some questions for you.

How Many Emails Can I Send Without Getting Blacklisted?

It’s not just the raw amount of emails sent that can get you blacklisted.

Email frequency and suddenly increasing volumes can affect you as a sender and make your emails go to spam.

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

If you suddenly start sending a ton of emails compared to a baseline of relative inactivity, then there is a good chance ISPs will flag your IP address for suspicious activity. Email volumes need to be built up organically and not be sent too frequently in order to doge blacklist and spam filters.

This is how the senders’ reputation warm-up process looks like.


How Can I Tell If My Email Has Been Blacklisted?

The fastest and easiest way to check if your email is on a publically available blacklist. Common databases include:

These free tools will check your IP address with over 120 publicly available blacklists.

You also should keep an eye on email campaign stats, including opens and clicks but more specifically domain opens. If you notice a sharp drop off of domain opens, then that is a good indicator that you may have been blacklisted from and ISP.

Sometimes though you won’t be able to tell if you are on an email blacklist. 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 managing your email campaigns.

What Should I Do if I’ve Been Blacklisted?

Sometimes you can do everything perfectly and still find yourself on a blacklist.

The most important thing is: Don’t panic. 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 seem to put a huge dent in marketing efforts, worries notwithstanding. Marketers who described their campaign as successful were just as likely to be blacklisted as other campaigns.

If you find yourself on a blacklist though, then that means you can probably step up your marketing game. Fortunately, pretty much every major blacklisting company has ways to appeal your status and have your IP address removed from the list. In general, the process of getting yourself removed from a company blacklist is pretty simple; Just contact the company and do what they ask you to clean up your email profile. Sometimes, a company might drop you from the blacklist on its own if complaints on your IP address drop off.

If you are using an ESP, they usually take care of fixing this problem. They will usually contact the companies to remove the IP blocks but your account may be suspended until you comply with their demands.

How to Avoid Being Put on a Blacklist

The best way to handle blacklists is to avoid being put on one in the first place.

Keep your email list curated

Bad email lists are the number one culprit for being put on an email blacklist. For instance, you should never buy an email list as those addresses are likely not legitimate. In the worst case, they can be spam traps which will almost ensure you are put on a blacklist. You should remove any bounced email addresses from your list and remove old ones that are not performing well. Remove any emails that have not seen much activity and make sure you honor client requests to be taken off 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 they can click on to confirm their email subscription.

Double opt-in schemes are good for avoiding email fraud as they ensure that the user is a real person and not a bot.

Double opt-in schemes are much less prone to complaints too as customers won’t find themselves on a mailing list unless they want to be on it for sure.

Protect your server

You also need to make sure your server is clean from any malware or bots. These harmful programs hijack your email domain or IP address to send bogus emails, so you can end up on a blacklist even if you haven’t done anything wrong. These programs can infect your server without your knowledge so make sure your server security protocols are up to date.


Unfortunately, pretty much every company will have their email put on a blacklist at some point; it’s just an inevitable part of running a business. Blacklisting is not permanent though and there are several things you can do to prevent getting put on a blacklist. The key thing to remember is that good email etiquette will drastically reduce the chances of being stuck on a blacklist.

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This article was originally published by our friends at Omnisend.

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