Email Blacklists - What You Need to Know

How Black Lists Work?
Vitalii Poddubnyi Vitalii Poddubnyi 15 february 2022, 14:29 1059
For beginners

What is an email blacklist and how can you avoid it? Dive into email blacklist prevention tips to boost your delivery rates with our UniOne guide.

What Is an Email Blacklist?

A blacklist is a list of IP addresses and domains that seem suspicious to email servers or, though not necessarily, have been blocked due to malicious actions, such as sending spam. The purpose of blacklists is to reduce the number of unsolicited emails to users.

If the sender’s domain or IP address is blacklisted, their emails may not be delivered at all or will pass through additional spam filters. Blacklists use different spam identification algorithms. For example, if the mailing list gets too many spam complaints from recipients, then the email delivery rate to the inbox folder will be reduced to zero — all further messages are going to end up in spam.

The percentage of blacklisted IPs in different countries

The percentage of blacklisted IPs in different countries (according to ReturnPath)

Blacklists are managed by blacklist operators, usually third parties, who collect information about all email senders. That information can then be used by a variety of organizations and companies, such as:

  • Internet Service Providers
  • Email Service Providers
  • Contractors specializing in spam protection, etc.

Depending on the type of blacklist and the organization that uses it, getting into one can result in either refusing to allow receipt of blacklisted emails entirely or sending them to a spam folder.

This is quite unfortunate if your work relies on email or if you send out a cold email.

An email blacklist, which is sometimes referred to as a single entity, is in fact a collection of many blacklist databases, all of which set their own standards for email delivery practices. Most widely used public blacklists include the Composite Blacklist (CBL), the Exploit Blacklist (XBL), the Spamhaus blacklist (SBL), etc.

These three blacklists are maintained by an international organization The Spamhaus Project. Email service providers such as Gmail, Outlook, and Yahoo consult with The Spamhaus Project to ensure that their users receive only legitimate and trustworthy emails in their inboxes. For example, if you use Gmail, you might see a notification "This message seems dangerous". Private companies can also create their own internal lists to blacklist email addresses or domains and stop those from contacting them by email.

These various blacklist databases function by collecting the reputation scores of IP addresses or domains. These scores can be adjusted, for example, when a user marks a certain email as spam, or automatically calculated based on factors such as bounce rate, spam trap detection, engagement or deliverability rate.

Types of Email Blacklists

The most widely used services for blacklisting

The most widely used services for blacklisting (according to ReturnPath)

There are different views on how blacklists are differentiated. Some divide them into two groups: IP-based and domain-based.If you end up in the first group, there’s a high chance that all the mailings coming from your IP will dointo spam. If it’s the second one — all mailings on behalf of your domain will meet the same fate. Among dozens of blacklists out there, the most influential ones are:

IP-based lists:

Return Path Reputation Network Blacklist (RNBL) — this list of IP addresses is maintained by Return Path. The company determines whether to block the sender, based on its data from the Return Path Provider Network. The algorithm is complex: these are prediction models, spam trap data, and complaint scores.

Spamhaus Block List (SBL) — this is used by nearly all email services. Blacklist algorithms check their databases and add email domains blacklisted for email policy violations to the list. Technical support works promptly. With this service, you can clarify why you have been placed on the blacklist. If you report a solution to the offending problem, technical support will quickly remove the domain from its database.

Exploits Block List (XBL) — is a list of known open services and illegal third-party exploits for sending spam and viruses. An exploit is a computer program that attacks computing systems through vulnerabilities. XBL also includes Spamhaus information.

Composite Blocking List (CBL) — is a blacklist based on DNS records and operating with IP addresses suspected of sending spam and virus infection. CBL obtains their data from large email servers and their spam traps. CBL offers a simple option for delisting your IP addresses.

SpamCop (SCBL) — this has grown to a complaints service where the user reports a suspicious mailing list. The service checks it, adds it to the blacklist if it is spam, and notifies the sender of the sanctions.

Passive Spam Block List (PSBL) — is a list of IP addresses from which emails have been sent to spam traps. PSBL also has whitelists, and if the IP address is in such list, then it is less likely to get into the blacklist.

Domain-based lists:

Uniform Resource Identifier Black Listing (URIBL) — this follows strict policies, a few hits in the blacklist, and your domain will remain there forever. It will be very difficult to get out. However they blacklist only for obvious violations, so URIBL is valued among email providers.

Spam URI Realtime Blocklists (SURBL) — is a list of websites that show up in spam emails. The domain owner can request the removal of a domain from the blacklist by following the delisting instructions.

Spam and Open Relay Blocking System (SORBS) — a popular blacklist which often blacklists newly registered domains. There are a lot of spam traps in the service. Friendly technical support will help fix the problem and then remove the address from the blacklist.

Weekdays with the biggest number of blacklisting

Weekdays with the biggest number of blacklisting (according to ReturnPath)

If you end up in one of these lists, the issues with reputation and deliverability will be significant — up to the point of full blocking and rejection of your emails.

We can also divide the whole range of blacklists into three parts.

1. Public Blacklists

These blacklists are the most dangerous ones, due to their popularity with spam filters and email providers. What happens if a domain or IP address lands in it? Emails can be blocked on different domains. Say you are on the SpamHaus blacklist — the sanctions will be impressive, since all emails on such email servicesas Outlook, Hotmail, Gmail, will be rejected.

2. Enterprise SPAM Firewalls

This is a subset of the public blacklists group and is supported by the spam firewalls of individual corporate IT departments. A firewall is a network security device that monitors incoming and outgoing network traffic and, based on an established set of security rules, decides whether to allow or block specific traffic. Some of these are McAfee, Cisco Ironport and Barracuda firewalls.

Firewalls have been used as the frontline of network protection for over 25 years. They put a barrier between secure, controlled internal networks and any untrusted external network that is full of hidden dangers.

According to Barracuda, 83% of email attacks impersonate well-known brands and another 6% are impersonations of people, meaning nearly 90% of all email attacks rely on deceptive sender identity. A huge number of unsolicited messages causes significant harm to recipients. Enterprise SPAM Firewalls are used to fight spam, viruses and other malicious content distributed via email. One such device has the ability to process up to 10 million emails per day.

3. Private/ISP Blacklists

Many companies maintain their own lists, both white and black, which include legitimate and unwanted senders of email newsletters, correspondingly.

Private lists are less dangerous, since their maximum restriction is a ban on email delivery to a particular domain. Such problems can be solved in a constructive dialogue with the email server’s administrator.

What to Do if You’re on a Blacklist?

So now you have a blacklisted email, how do you get removed from the list, you ask. Well, whatever happens, do not panic and do not change your IP or domain. This will make things worse. This way you can ruin your relations with both blacklists and email providers. Email blacklist removal is not always an easy task.

First of all, find the cause of blacklisting. Check whether the latest emails were sent out to the right contact base segments. Also, go through the complaints about these emails and see if the rate is higher than usual. If the IP address is blocked and it is shared on the platform, then it is crystal clear that this IP address is used by other clients of the service and they are working poorly. In any case, it is worth contacting ESP technical support to resolve this issue.

As a rule of thumb, each blacklist has an FAQ section that describes possible blocking reasons. Before contacting, study it carefully and fix all potential problems. So the first step implies that you:

  • have conducted an initial analysis of the situation;
  • found out that it is indeed your domain or dedicated IP that is being blocked;
  • eliminated the reason for blocking.

The second step is to contact the blacklist directly and ask to delete your domain or an IP address from it. As soon as you receive an answer, make sure the situation doesn't happen again.

Keep in mind that not all blacklists will meet you halfway and remove you from the database. Also, if you have been removed, don't get your hopes up. You can still get blacklisted again. If the reason for blocking turns out to be the same, delisting will probably be more difficult. If the sender repeatedly requests whitelisting without making the necessary changes, their request will be automatically rejected.

If you ask yourself “Is my email address blacklisted?”, then the easiest way to find out what the problem is is with these most popular blacklist operators:

If you’re still asking yourself “Is my mail server blacklisted”, you can check it on Just visit the website, send a test email from an email address that you fear may be on the blacklist, and in a minute you will receive a report on which blacklists you are on (if any). It looks like this:

Checking if a mail server is blacklisted on

And all the necessary details are listed and explained below:

Necessary details about Checking if a mail server is blacklisted

An IP address can be removed from blacklists automatically soon after the reason it ended up there is eliminated. For example, if there are no emails for 30 days from the address that has previously shown suspicious activity, this may be enough to delist the address. But this happens quite infrequently— in most cases, it is necessary not only to address the reason for being blacklisted, but also send a special request to remove the address from the blacklist of a particular database.

How Do You Avoid Blacklists?

There are several measures that can help you stay away from blacklists.

Organize the subscription of new users via a double opt-in procedure

With double opt-in, the user is first sent an email with a link for address confirmation. Thus, spam traps do not make it into the address list, the open rate and click rate increases, unsubscribes and spam complaints are reduced, and the ROI for a given campaign will increase.

Clean the database of invalid addresses from time to time

Subscribers may lose interest in your newsletters, this is normal. If you see that part of your audience does not show any activity or even complains about spam, set up a campaign to reactivate subscribers. Those users who continue to ignore your emails should be removed from the list, otherwise over time this may cause some bad effects on deliverability.

Use separate IP addresses and domains for different types of mailing

Do not send all emails from the same domain or IP address, because if they are blacklisted, the deliverability of all mailings will suffer. It is even better to use separate servers for different purposes. For example, you can send promotional emails from one IP address, and transactional emails from another.

Provide an opportunity to quickly unsubscribe from the mailing list

To reduce the number of spam complaints, give subscribers the opportunity to unsubscribe from the mailing list without unnecessary actions.

Be consistent

Scoring services love to see beautiful, natural, organic activity. If you instantly go up from sending 10 emails a day to sending 10,000 emails a day, that's wrong. Sudden or suspicious changes in your email activity may put you on the blacklist.

And finally... never ever buy mailing contact bases!

Think about it — buying a list of randomly chosen email addresses of people who have probably never heard of you and definitely did not agree to receive your emails. When could that be a good idea? Right, never. We'll shout it from the rooftops if necessary — do not purchase email lists!

How to Protect Your Business Email Account?

Sometimes, despite following all the rules, you can still land on a blacklist. However, it is important to take all measures to avoid this. If you want to protect yourself from accidental blacklisting, set up a dedicated IP address to move there at some point. That will cost you a certain amount of money but it will be exclusively yours. There may be several users on a dynamic IP address, and getting one of them on a blacklist may negatively affect all the others.


If you want to learn how to send bulk email without getting blacklisted, you need to be patient and attentive. In order to maintain the high reputation of the sender, we recommend regularly checking blacklists and analyzing your mailing lists. When using email services, always follow the rules of these services, especially their anti-spam policy so that your emails reach the inbox. Keep in mind that behind these letters and code strings there are humongous machines that scan millions of emails everyday and are not that easily tricked.

Focus on the quality of your content, monitor the technical settings of emails and use all providers’ features to conduct effective email marketing campaigns.

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