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Last updated: 01.03.2024 with new content on email warm-up

At hubsell, I advise customers to avoid spammy outreach behaviour to not get yourself into an email domain blacklist. There are several ways to minimize the chances of getting into that position. For example, sending messages at a human level pace.

Unfortunately, there are salespeople that do not know of all the guidelines. So, in this post, I want to share with you what is an email domain blacklist and its different types. Also, you may be asking yourself, is my ip blacklisted. So, I will explain what can cause your domain to get blacklisted and what to do in that case.

Before I start breaking down each topic, here is the list of them for reference:

  1. What is a blacklist and its effect on inboxing
  2. Different types of blacklists and examples
  3. What causes a domain to get into a blacklist
  4. How to avoid getting onto a blacklist
  5. How to know if you are on an email blacklist
  6. What to do if you are on a blacklist and how to get off one
  7. NEW: What is email warm-up and when & how to use it
  8. How to use the contents of this blog post

What is an email blacklist and its effect on inboxing

Every day, there are tens of billions if not hundreds of billions of spam emails sent per day. With the number of spam emails sent dwarfing the number of legitimate emails that are sent it’s no wonder Internet Service Providers (ISP) and other organization work so hard to prevent spam from being spread around.

With such a large number of spam emails sent, blacklists are created to try to combat this and block IP addresses and domains that are known to send spam.

Whenever an email is sent, the IP address is contained within it. An IP address is a unique string of digits used to identify the device or the server an email was sent from. Similarly, domain names are the website’s name and can also be used to identify the website or organization is was sent from e.g.

An email blacklist identifies and records, domains or IP addresses that have been sending spam, as per the standards of the blacklist’s owner. ISPs and mailboxes use them to protect their users from spam.

Here is the process whenever you send an email:

  1. you send your email as a Sender to the Receiver
  2. the email is received by an ISP who analyses it
  3. the ISP checks if your domain is on the blacklist that they are using
  4. the spam filters of the ISP evaluate your email
  5. if your email passes the tests, then it is inboxed

In conclusion, your email can be rejected if it does not comply with the ISP’s policies and/or if your domain is on the blacklist in use. The outcome of the analysis determines if your email will be inboxed or sent to the spam folder.

Now that I have clarified the definition of a blacklist and how it affects inboxing, let’s explore the different types of blacklists and a few examples.

Different types of blacklists and examples

There are two types of blacklists:

  • IP address blacklists – a collection of IP addresses known to send spam emails
  • domain blacklists – a collection of domains identified as senders of spam

IP address blacklists

There are two main types of IP blacklists: Real-time Black Lists (RBL) and Domain Name Server Black Lists (DNSBL). They update their databases in real-time. ISPs and mailbox providers can use those blacklists to see if the sender is flagged as a spammer on that list. The common blacklists of the RBL/DNSBLs type include:

Domain blacklists

The domain blacklists, such as URI Real-time Blacklists (URI DNSBL), are lists of spam domain names that can appear within the email body. Email providers can use the blacklist to analyze and check for spam domains in the main message. The most commonly used URI DNSBLs include:

Let’s now examine the reasons for what causes a domain to get into a blacklist.

What causes a domain to get into a blacklist

With the immense number of email sent every day, it is totally impossible for a human to check if an email is spam or not. So what actually happens is there are anti-spam techniques and spam filters that have a criteria to check whether the email is spam or legitimate.

Knowingly or unknowingly, if you send an email and trigger some of these criteria, you can find yourself getting blacklisted – and you’ll be surprised at how little it takes to be blocked from sending emails.

There are many ways that your domain can end up on a blacklist, but it often has to do with email. Here are a few causes of a domain getting blacklisted:

Receiving spam complaints

If the recipients of your email flag it as spam, then the mail providers will think that it is true and it is not of value to the prospect. Even if your list is clean and opted in, a click on the ‘mark as spam’ button will count as a spam complaint. The more complaints you receive, the more likely it is for your IP or domain to be added to a blacklist.

Acquiring bad email lists

When your campaigns have a high bounce rate (above five percent) due to bad quality lists, the ISPs think that your list may not be opted-in or current. Also, if you send emails to spam bait addresses, then that will get you onto an email blacklist.

Sending identical emails

Whenever you send close to identical emails to lots of people, that also triggers the spam signals. Emails that are not personalised are categorised as spam with no valuable content for the recipient.

Abusing daily sending limits

Companies that send a big amount of cold emails per day will raise the alarms for ISPs. Also, if the interval between those emails is non-existent or identical, then it will also be considered spam.

Spam traps

Email addresses that are inactive for long periods of time, invalid or have never had any email engagement are monitored and used to identify senders of spam email. The reason being is that the lists being used are extremely outdated or clearly demonstrate the sender is mass sending very poor quality emails.

If you avoid making the above mistakes, then the chances of getting blacklisted will be close to zero. Let’s next examine how to avoid getting onto an email blacklist in the first place.

How to avoid getting into an email blacklist

The ideal scenario is for your domain to not land on an email domain blacklist when doing cold outreach. Here are a few ideas to keep your domain out of blacklists:

Avoid spam email content

There are keywords that are highly associated with spam and should be avoided. Some words include ‘free’, ‘money’, ‘risk free’, ‘apply now’, etc. Furthermore, the use of capitalizing words screams spam as well as excessive use of exclamation marks and other punctuation.

Get higher email engagement

Email engagement, which is email open and reply rates, is huge in determining whether an email is spam or legitimate. Since nearly all spam email will have a very low email engagement, if you are getting really low email metrics, it may appear to an ISP that you are sending spam.

Writing self-focused and salesy emails will receive spam complaints in the B2B world. Instead of talking about how good you are, shift your focus to the problem-solution dynamic and how your product fits into it. The prospect should gain value from the email without having to buy or start a conversation with you.

Watch your email campaign’s bounce rate

A quick way to get your domain on an email blacklist is by having a high bounce rate. So, keep track of it and maintain it ideally under five percent. Ensure you have a process of regularly validating the data and checking to see if the email addresses are still active. In your campaigns, remove the bounced addresses to avoid sending follow-ups to them.

Do not buy lists

Avoid buying email lists. They are usually infested by old and irrelevant emails or spam baits from blacklists. Those lists will lower your campaign’s conversions and prove to be more expensive in the long run.

Instead, opt for reliable on-demand quality data partners. They can provide GDPR adherent data that is verified and without bad email addresses.

Personalise your message

Yes, this takes time and that’s the point. You don’t want to be sending the same templated message to your prospects – as that is what a spammer would do. Through your research, you should personalise at least the first line of your message writing something unique about them.

Not only will this show the ISP’s that each message is unique, but your email engagement is likely to increase too as your prospects see you took the time to research them first.

Segment your lists

To ensure that your prospects are getting relevant email copy, separate them onto different lists. That allows you to go one step further into email personalisation, which avoids spam complaints.

You can create entirely separate campaigns targeting different levels of seniority, industries, departments, or even geographical locations. Each will have different pain points or business needs that you can tailor your message for.

Adapt your sending volume and intervals

If you send too many emails per day at the same time, then you are likely to end up on an email blacklist sooner or later. So, keep the number of sent emails below 100 per day per mailbox. Ensure that you the interval between email are not a fixed amount of time e.g. exactly 30 seconds apart from each other. Fixed sending intervals is a very obvious way of saying that you are automating the sending of your emails, it is better to send using random time intervals.

The general recommendation is to try to look as human as possible in your campaigns. It should seem that you are a person sending emails to friends and colleagues manually.

Now that you know how to avoid email blacklists altogether, let’s examine what you can do if you find yourself on one.

How to know if you are on an email blacklist

You may be asking yourself, is my IP or domain blacklisted? Before we get to this, you first need to know the different types of blacklists that you could be on.

There are hundreds of email blacklists out there of all different sizes. Most can be checked on public blacklists. The following tools will search the majority of the major public blacklists and help you to see if you are on them.

What to do if you are on a blacklist and how to get off of one

Assuming that you are not actually a spammer and that you have been following the email marketing best practices, it can be very frustrating and damaging to your business if you are unable to send emails.

Blacklists are useful to protect people from harmful or non-relevant content. But they are not perfect. Even if you are on one, do not be alarmed because, usually, you are removed after some time and there are no effects afterwards.

If your domain is on a smaller blacklist, then it will probably not affect your cold outreach because few ISPs use it. In that case, you do not have to worry because your IP will be excluded sooner or later. On the other hand, larger lists are commonly used by ISPs which can impact your inboxing rates.

So, to remove your domain/IP address from those blacklists, it can be as simple as contacting its owner and requesting to be removed. The major blacklist companies have information on their sites on how you can remove yourself from their lists. Usually, the process is straightforward.

However, some blacklist operators may ask that you prove to them that you have improved your email marketing practices and corrected the reasons you were first blacklisted for.

Finally, to avoid getting onto other blacklists in the future, make sure to follow our guidelines from this post. 

What is email warm-up and when & how to use it

Email warm-up is a set of techniques used to artificially create a reputation of mailbox for the purposes of using the mailbox for sending out high volume of emails. 
The way it works is you connect your account to a email warm up services provider.  The email warm up service provider sends a number of emails from your to-be-warmed up account to various email mailboxes that the email warm up service provider controls. 
The emails that are sent from your account to these mailboxes are marked by the email warm up service provider as important, removal from ‘promotions’ folder to inbox, and marking your emails not-spam (in case they landed in spam folder). 
Further, the email warm up service provider also sends replies to these email to simulate real conversations. All of that is done to imitate two email traffic from your email and the various mailboxes that the email warm up service provider controls.
“Wait, but why?” you must think right about now. 
Well, here’s the reason. All the following actions such:
  • marking your emails as important
  • moving your emails from promotions to inbox folder
  • marking your email non-spam
  • replying to your email

are technical signals that email service providers (such as Google, Microsoft, Yahoo, AOL, 1und1 and a host of others) track across accounts. That means that if you are marked as spam on one Google account it is tracked and can be used to determine your overall spam rating. The same is true for positive actions such marking your email as important.

The more positive actions you get the better it is for your mailbox, however it is also important not to over do it. Email service providers can also detect when you are trying to juke the system so gradual increases of volume are recommended.

When would you use email warm up

There are two situations when you are recommended or even compelled to use email warm up techniques or services.
  1. You have damaged your mailbox’s deliverability
  2. You are creating a new mailbox for the sole purpose of outreach
Let’s unpack these two.
Situation 1, describes a mailbox which is your primary mailbox, you are using it to receive all the inbound mails you get form customers, colleagues, suppliers and host of other senders. There is back and forth traffic, and it is quite apparent to email service providers that the mailbox is operated for day to day business matters. 
BUT, you decide to this mailbox for high volume outreach with generic text and bad data. You get double digit in bounce rate. You get double digit in unsubscribe rate. Your open rate is a measly 20% and you are sat there wondering why am I not getting any replies form the 2000 contacts I blasted this week.
Well the answer is that you have been marked as SPAM. Your emails call their new home the SPAM folder. Your prospects never see your email.
Sounds familiar?
Here’s what you do:
Deploy an email warm up solution (many to choose from), but don’t stop there. If you don’t fix the issues that got your there you are dousing one fire out while creating another one.
Do the following as well
Fix your data quality and bring that bounce rate below 5%.
Iterate on your open rates and make sure you avoid SPAM words.
Personalize the content to the recipient.
Spread out the sending over a period instead of bulk sending.
Situation 2, is much more straightforward. You want to add a new mailbox under your name. For example your main email address is, and you want to add another mailbox with Where you continue using your main mailbox for your day to day communication and you use the new mailbox for outreach only.
In this case it is recommended to do two things. 1) Warm-up the mailbox if you plan on hitting high volumes right from the start and 2) make sure you follow this whole post well and also read our post titled Everything you need to know about email deliverability and open rate
How to use the contents of this blogpost

In this post, I discussed what blacklists are, how to avoid them, and how to get removed from one. Hopefully, you can take away something from these lessons to minimize your risk of getting into one. And remember, getting on an email blacklist is not a big deal, you can remove yourself most of the time. Here are the main takeaways of this post:

What is an email blacklist and its effect on inboxing

An email blacklist identifies and files domains or IP addresses that have been sending spam, as per the standards of its owner.

You send your email as a Sender that is received by an ISP who analyses and concludes if your email is spam or not. If your email passes the test, then it is inboxed.

Different types of blacklists and examples

  • IP address-based
  • Domain-based

What causes a domain to get into a blacklist

  • spam complaints – too many people rejecting or unsubscribing from your emails; can be due to bad targeting or general emails
  • bad lists or addresses – a high bounce rate because of poorly sourced email prospects will put you on an email blacklist if you are not careful
  • identical emails with no personalisation – similar emails will trigger the spam signals from the email providers which will send them to the spam folder
  • spam sending rates and volumes – sending too many emails per day without enough or varied interval between is how you get into a blacklist

How to avoid getting on an email blacklist

  • write email content that resonates
  • watch your email campaign’s bounce rate
  • do not buy lists
  • segment your lists
  • adapt your sending volume and intervals

What to do if you are on a blacklist

Identify which blacklist you are in, visit the website of the owner and follow the procedures to get yourself removed. Be patient and hopefully you will be out soon.