What are Email Blacklists? 4 Surefire ways to keep your Domain off them!
Written by
Tejas Pitkar
Tejas Pitkar

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What are Email Blacklists? 4 Surefire ways to keep your Domain off them!

Published : May 26, 2020

Every email marketer has experienced having their sending domain/IP being blacklisted at least once in their marketing tenure. Depending on which email blacklist your domain/IP lands up in, can either be a non-issue or a complete show stopper to your email marketing activities!

Your email deliverability and domain/IP blacklisting are related to each other hence the negative impact can be disastrous for your email program. If you are a part of an email service provider then the email deliverability consultant’s job is to keep you off the email blacklists.

But it is important for a brand marketer to be thorough on their knowledge of how blacklists function as it is intrinsically linked to your email inboxing and performance.

In this post we have covered the basics of what are these email blacklists and how to avoid them. Also, we share some tips on what measures you can take to resolve the issue if your domain/IP lands up in blacklists.

What is an Email Blacklist?

Any email blacklist as the name suggests is created to mark the senders of spam, with their IPs/domains getting flagged for sending unwanted messages to unsuspecting users. It is a list curated during real-time that identifies the sender’s IP/domain as a spammer and hence those emails are blocked.

Email Blacklists are maintained by mailbox providers like Gmail, Yahoo, Hotmail, Outlook, etc, as well as independent spam filtering companies that are created with the sole purpose of detecting these spammers and restricting their email delivery.

Blacklists help to keep unwanted spam messages out of a user’s inbox and help the mailbox providers (MSP) to deliver a good user experience. Large MSPs like Gmail maintain their own blacklists based on the data they have collected on bad sender IPs and domains. But other MSPs also rely on these independent spam filtering agencies for referring to their blacklists. These agencies maintain something called as DNSBL(Domain name system based black lists)

The way an MSP processes your email is first checking their internal/external blacklists for IPs/domain and if they verify that you are an authentic sender of emails then they further allow their anti-spam filters to process your message content. Thus, the rules vary across MSPs to consider email blacklists in making inbox placement decisions.

So this brings us to what are some of the top email blacklists to monitor and check if you have landed up in them.

Types of email blacklist companies

The independent email blacklisting companies keep track of real-time email delivery globally and if your brand is following any bad practices, the likes of which we have mentioned later in this post then they will add your IP/domain to their blacklist.

Here are some of the blacklists which are considered serious for your IP/domain to appear in them:


Spamhaus mostly uses their own global database of known IPs and domains which are spammers. They also create these lists with their own planted spam trap ids which are mostly found in purchased lists or web-scraped third party lists.

The following are the types of Spamhaus blacklists:

  • SBL (Spamhaus blacklist) : They contain IPs which are collected in real time across the world sending spam email. They also create a list of URLs which have been blocked in the past or are suspicious.
  • DBL (Domain blacklist) : They create a list from domains with bad reputations in the past of sending emails from the world over.
  • SBL CSS : This blacklist is linked to DBL as if your IP gets blacklisted in this list means the relevant domain has been listed by DBL. In this case, both DBL as well as SBL CSS need to be whitelisted so that you get off the blacklisting.
  • PBL (Policy blacklist) : These contain IPs which are matched to the ones under consideration which should not be delivering unauthenticated emails to any other internet mail servers except those provided by their ISP.
  • XBL (Exploit blacklists) : These contain IP address ranges of hijacked computers which contain worms, known trojans, viruses etc.
  • Zen blacklist : Zen is a combination of SBL , PBL , SBLCSS , XBL blacklists.


Spamcop works on reporting and blocking. Both function independently of each other. Reporting part allows any user to file abuse complaint with admin about spam. These reports are then send to blocking service to keep it updated and block that IP/domain. The block service is made available to mail admins to use as a standard DNBL. Spamcop will crop the headers of unwanted email and send a complaint of the same to system admin responsible for spammer’s internet access.


The Invaluement blacklist is made up of three commercial anti-spam blacklists.

  • ivmURI: This lists URIs like domains and IPs owned by spammers.
  • IvmSIP: Spammy IP addresses either overlooked or not yet listed by Spamhaus,from botnets, elusive snowshoe spammers, or black-hat mailbox providers. These are the ones who mail to recipients from purchased lists where no permission has been taken to send them any kind of email.
  • IvmSIP/24: IP addresses or sub-networks where spam patterns have been found.


SORBS (Spam and Open Relay Blocking System) is a DNSBL list is made up of IPs/domains associated with sending spam. It also includes servers which have been hijacked or infested with trojans,viruses etc.


Baracuda central networks detect the threats coming to their system from various mail servers. The IPs and domains consequently are listed and a user can do a lookup on their website. There is a BRBL (Baracuda reputation blacklist) which aims to list bad sender IPs which are collected by their automated system.

If your domain appears in any of the blacklists mentioned above then your email deliverability will take an immediate hit . The impact of keeping on sending emails despite being on blacklists has far reaching negative repercussions.

Your domain in such cases can get blocked permanently by the anti-spam bodies which will be a show stopper for your email program.

Apart from these independent email blacklisting companies, there are the ISPs who run their own blacklists based on the history of data they have collected on senders and IPs.

So let’s see how they function.

How do ISP email blacklists work? 

ISPs stand for email service providers like Gmail, Yahoo, Outlook ,Hotmail which are email clients for recipients.

They have their own blacklists and blocking policies eg: Gmail’s anti-spam filtering system is the most advanced in using AI/ML techniques to filter the incoming message content and block it if it is dangerous. Nowadays, even small MSPs, with the help of AI/ML,big data store their own data on bad senders and have their own blacklists.

ISPs will blacklist your sender domain if you have followed any bad practices and have not showed signs of being a legitimate sender. Ending up on a blacklist for your IP or domain is a directional indication of what’s going wrong with your list hygiene.

Mailbox providers blacklist the sender domains/IPs on the basis of the following:

  • Continue email activities despite the sender domain having bad reputation.
  • Mailing with delivery IPs which are red in Google Postmaster, i.e. they are bad reputed.
  • If your emails incur a huge amount of reported spam complaints from users, this could have an immediate effect on your future emails filtered into spam.
  • If your message body contains suspicious, fraudulent links which would signal ISPs of the sender indulging into spam activity.
  • ISPs also have a weighted system where they can refer to independent blacklists mentioned above for IP/domains which are listed due to bad history of sending emails. They can provide spam points to these different blacklists and then based on the points threshold come to a conclusion on what is to be done with the email.

So the next time you see an error message in Gmail like “Lots of messages with this sender domain have been found to be spam”, you should know that Gmail has blacklisted your sending domain/envelope and you need to build reputation on it again to start inboxing.

If your brand uses an ESP to send emails on your behalf then in case of an ISP blacklisting, they should contact you on the same to de-list it and get your IP addresses unblocked.

But if you don’t have an ESP then it is up to the brand marketer to monitor all these blacklist issues and try to follow good practices to avoid blacklisting.

Which brings us to what are some of the bad practices that get your domain into trouble.

Bad practices that get your domain/IP into email blacklists

The following practices will land your domain/IP into trouble with blacklists:

Mailing to spamtraps

Spam trap is an email address used by a blacklist agency which is not operational but is used in harvested or purchased lists. The email id is planted in such bad lists to figure out if the sender is using a purchased list and is violating good practices of permission marketing.

With IP blacklisting the thing that is a misconception is that IP getting blacklisted means an email was send to one of the spam trap ids being monitored. But the reality is that your IP only gets listed when you have potentially hit hundreds of spam traps.

Thus if you are mailing to spam traps then there is something seriously wrong with your email program.

There are 3 types of spam traps:

  • Recycled spam traps: These are spam trap ids which have been inactive on email for more than 1 year and are now used by ISPs to act as spam traps.

They are able to detect violators and spammers for independent blacklisting as well.

  • Typo based spam traps: These are spam trap ids which turn up on a sender’s list as there are obvious spelling errors in the email ids. Once these typo email ids receive any emails, then it indicates the ISP that the list acquisition and list maintenance is poor.
  • Pristine spam traps: These email addresses are active but never used to sign up for any subscription based service. As a result when these start receiving any emails, the sender turns out to have purchased the list or scraped it off the internet i.e. harvested it.

Spam traps are like honey pots as they provide essential information to ISPs and blacklisting agencies on a sender’s profile and whether they are sending emails to their subscribers only or spamming to purchased ids. They are set-up to monitor malicious activities.

High amount of user spam complaints

If your sending domain incurs a spam complaint rate of >0.5% on Google Postmaster then that indicates to the ISP that a majority of your users do not wish to receive your emails and have reported them to spam. The more regular your spam complaints, the higher the probability that your IP lands in blacklists.

Incurring regular hard bounces

If your mailing list contains a lot of typos in email addresses and if those are mailed to, they will bounce back. Hard bounces occuring on a consistent basis, affect your email deliverability and domain hygiene. Thus, if the hard bounce percentage of your mailing list is greater than 5% on every send then the blacklists will consider this as a best practice violation with your list acquisition practices will be under question.

Using new list source sans validation

Blacklists can also be triggered due to a result of adding a new list source and not validating the email addresses of the list, neither confirming the list acquisition source.

This results in high bounces and spikes in spam traps hit as the new ids being mailed could have been harvested off the internet.

Using malicious links in your content

This is usually a spammer practice where the underlying links in the body content lead you to a different landing page which is not relevant to the email.

Spammers try to use several fraudulent techniques like phishing, spoofing , worm attacks , MiM attacks to commit cyber-crime. Thus, ISPs and independent blacklists scan your entire mail content to find these suspicious links.

Use of common links in your email campaign with those that have already been monitored in “spam” emails will get your domain blacklisted.

If they are found to be malicious then ISPs generally do 2 things:

  • Flag your email as dangerous and tag it into spam.
  • Block the email delivery and blacklist your domain.

Ensure that the links you are including in your content are the ones you trust and is associated with your brand.

Running re-engagement campaigns randomly

Re-engagement campaigns are run by marketers in order to win back their inactive users who have been ignoring their emails for a while. These are the dormant users who have not been touched base for the last 1 year or so.

The users from this list could have easily been converted into pristine spamtraps and can be used to detect a sender’s bad list maintenance. It is often observed that running a re-engagement campaign to a large list of inactive users causes a spike in spamtraps hit and bounces resulting in landing the IP in blacklists.

These are the practices to avoid for getting your domain/IP blacklisted.

Now how do you know if you have landed in any of the blacklists?

How to know if your domain/IP has landed up in email blacklists? 

There are ways to figure out if your infrastructure has landed in any email blacklists either ISP or independent. Sometimes it is a result of root cause analysis done by a marketer and other times it is an error message thrown by your ISP.

Most likely your network administrators inform you about blacklisting when the recipient server of your email throws back an error message of email rejection. You can also use an email blacklist checker tool to identify whether your domain or IP address is listed on any on the global blacklist databases.

The marketer will have to check if you have received an unusual spike in your hard bounces lately. Then go and check the bounce code errors for them and figure out if how many of those email addresses you mailed to are typos and spamtraps.

There are visual indicators in some ESPs to see how many common spam traps have you hit for your campaign.

Also check for your brand’s engagement ratio of number of opens to number of delivered emails for a particular segment. Low number of opens might also indicate that these were just recycled spam traps who receive emails but never respond. An email marketer should think about their segments and what kind of engagement they provide.

Some analysis on your email lists will likely give you insight on the faulty list collection practices that you may have due to which you might be receiving low quality email addresses.

On the other hand, MSPs like Microsoft provide errors mentioning that your IP has been blocked and which blacklist it has ended up on. That helps a sender for the de-listing process.

Practices to avoid being a part of email blacklists

Trying to solve a serious issue, is to acknowledge first that you have one. The following are the practices to avoid getting flagged as a blacklisted sender:

Maintain a good list hygiene

If you wish to stay off blacklists then take good care of your mailing list.

Avoid ignoring unsubscribes as they could be the trigger for reporting your emails to spam. It is mandatory to display an unsubscribe link for brands for the users to stop receiving your emails.

Your list needs to be segmented according to attributes for better targeting. These could be on the basis of their past transaction history or website activity.

There is also a need for moderately active users to be re-engaged. Those who have not engaged with your emails for last 6 months to be said goodbye and sunset from your list.

Your loyal list of active users needs to be engaged on a regular basis to keep your engagement high and drive 80% of your conversions from 20% of the database. Keeping a good list hygiene will go a long way in ensuring your IPs and domains stay off major blacklists.

Monitor list acquisition and email validation

When a user signs up to your email newsletter they need to know exactly what to expect from your brand and are their preferences being collected for mailing frequency.

Don’t scrape addresses from the internet or harvest any third party lists. Mailing to purchased lists is a sure fire way of getting in the email blacklists.

The email ids collected need to be validated with Google and social plug ins which is a common practice used now. It would be great if you can send a verification email to their email ids to confirm your subscription as that will ensure that you are getting genuine users who are active on email.

Mailing to invalid ids is a major reason for blacklisting to occur, hence make sure you are mailing only to valid email users. There are several email validation services to get that job done.

Monitor spam complaints closely

Often times marketers tend to check the engagement metrics in a detailed manner for their campaigns but, they fail to check the amount of spam complaints their domain is generating and the abuse being received from feedback loops. Google Postmaster provides that information for your gmail audience.

Even if you are getting a high percentage of opens for your campaigns but low clicks, and at the same time get a high spam complaint rate, it means that there is a relevancy issue with the content you are sending.

Spam complaints or abuse signify to MSPs that your content is either not relevant or you are spamming your users with uninteresting content. That could land you in blacklists quickly. Pay attention to different FBLs to provide you information on your abuse complaints.

Keeping your content relevant to your users and sending them the creative at the right time will ensure that your brand affinity remains intact and your users don’t churn out getting bored with your content.

Email active recipients for engagement

Your active users are your biggest ‘brand fans’. They open and click on your emails regularly, they do multiple transactions. Spread out your velvet rope for them with exclusive offers and offer them VIP access. This will increase your engagement even further, which is a positive indication for a MSP.

Mailing to such users will keep your IP and domain reputation high and there won’t be an issue in featuring in email blacklists or any failures in delivery.

The motto of mailbox providers and anti-spam agencies is clear – “Send mail to the right user at the right time with the right content.”


Email blacklists exist to counter spam and elevate user experience. The solutions for avoiding email blacklists can vary between brands.

The purpose of an email blacklist is to keep mailboxes clean and only deliver the emails to users who have signed up for it.

Sending emails to your genuine subscribers will help you avoid email blacklists and will have a domino effect in keeping your email deliverability high as well.

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