Top Blocklists - What You Need to Know

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(Article Updated - August 2024)

Overview

A blocklist is a database of IP addresses or domains that have been associated with the sending of unsolicited commercial email or spam. Internet Service Providers (ISPs) and business email networks use information from blocklists to filter out unwanted email. As a result there can be a drop to inbox delivery rates if the IPs or domains involved with sending email are listed on a blocklist.

Marketo’s Email Delivery and Compliance team monitors blocklist activity on our IPs and domains daily. When an impactful listing occurs, we reach out to the blocklist, attempt to identify the sender that triggered it, and work with the blocklist organization to get the listing resolved.

There are thousands of blocklists, most will not have a significant impact on your delivery rates. Below, we have compiled a list of the blocklists that commonly appear across our sender ecosystem. Each blocklist has been grouped into a top tier (Tier I) by most impactful, to the bottom least impactful tier (Tier III). These may have little to no impact across our sender network.

Tier I Blocklist

Spamhaus (SBL)

Impact: Spamhaus is the only blocklist that we categorize as a Tier I for a reason: it has by far the greatest impact on delivery. It is the most well-respected and widely used blocklist in the world. A listing at Spamhaus will have a negative effect on your ability to deliver emails to your customer’s inbox and can cause bounce rates of over 50%. Evidence suggests that most of the top North American ISPs use Spamhaus to inform blocking decisions.

How it works: Unlike many blocklists, Spamhaus lists senders manually. This means they are proactively watching sender activity, collecting data, and base listings on a number of variables. Most commonly senders are listed for mailing to spam trap addresses that Spamhaus owns. Sometimes Spamhaus will list senders based on recipient feedback as well.

Next Steps: Please note that every Spamhaus email can be quite different depending on the information provided and the nature of the listing . Because Spamhaus has multiple types of listings, the remediation steps are based on which type of blocklisting has occurred. We have listed the most common types to affect Marketo customers, from most to least impactful. Impact can range from a full block of top severity, to a partial block that is less severe (CSS Data, DBL). If remediation isn’t performed and the problem not addressed, these listings can increase in severity and turn into a full blocklisting.

Spamhaus Blocklistings Types

SBL (full blocklisting) Considered the most impactful.

Remediation steps: Our team monitors closely for Spamhaus listings. Once alerted to the listing, we send an email notifying the customer, and reach out to the Spamhaus contact to start the remediation process. Only a Marketo delivery team member should be in direct correspondence with the Spamhaus contact, to speak on behalf of the customer, and to relay their questions and instructions, ensuring quick resolution and reduced impact. Senders that trigger this listing on a shared IP range will be moved to a more isolated, penalty range (IPB), so as not to impact the other shared senders. Those affected on a Dedicated IP, only impact their own sending and will not be moved. However, depending on the severity of the issue, our team may need to revoke a customer’s ability to send any emails until full resolution. The listing will last until Spamhaus is satisfied that the offending sender has taken the appropriate steps to mitigate the problem.

SBL CSS Customers are alerted of the listing with an email containing all the information needed to immediately begin the remediation process. This is part of an automated trigger listing, that allows for a customer to delist directly on the Spamhaus website, after they’ve completed remediation.

Remediation steps: Customers will go to the Spamhaus IP lookup website, at https://check.spamhaus.org, where they can check on the status of their IP and continue to monitor it in real time, until it is no longer listed there. Follow the remediation and delisting instructions provided, and check for specific details in the blocklist notification email.

DBL (Domain blocklisting) Customers will receive an email notification alerting of the company domain being listed. The email will contain specific information to help narrow down and identify which email source likely triggered it within Marketo. It’s important to note that any email sent from outside of Marketo, that contained the company domain, is suspect. This means, if the sender uses multiple IPs and/multiple email platforms, then any of those could be the source.

Remediation steps: Follow the detailed instructions in the email, which will also provide a link to the Spamhaus Domain reputation checker webpage, to check the real time listing status of the affected domain. Once the email source is identified and cause addressed, this customer will follow the online instructions to request to be delisted.

Tier II Blocklist

SpamCop

Impact: SpamCop is not used by any of the major North American ISPs to inform blocking decisions, but it makes it to the Tier II list because it can have a significant impact on B2B email campaigns. SpamCop is considered a Tier I one blocklist for B2B marketers but a Tier II for B2C marketers.

SpamCop is a dynamic IP blocklist, that can affect a single IP or a subset of IPs Typically, the block will automatically lift within one business day, but can take longer for relisted IPs. To have triggered a SpamCop listing likely means the sender has a list management problem that should be addressed.

How it works: SpamCop lists IPs for one of two reasons:

Either the email hit SpamCop spam trap addresses OR
A SpamCop user has reported the email unwanted.

Most of SpamCop’s spam traps are previously valid addresses that have not been active for 12 months or longer.

Remediation steps: If you are seeing a significant number of bouncing emails caused by a SpamCop blocked IP but aren’t sure if your email activity triggered the listing, first identify whether you are sending on a shared sender network or not. If sending from a shared IP range when this occurs, you or any other customer sending from the same network may have contributed to the IP block. This IP block will automatically get dropped within a business day.

For those on a Dedicated IP that trigger a listing, refer to the above remediation steps and resources, to address the list management issue.

Tier III Blocklist (Low/ No impact )

These are considered the lowest tier and therefore cause the least impact across the Marketo sender ecosystem. Some of these blocklists were more impactful at one time, while others are only impactful based on the sender region (Manitu). Others still can suddenly flare up (Lashback). There are also many blocklists that are ignored (0spam), and are not taken seriously because they do not provide any means to delist once on the list (NoSolicitado) or that they charge money to have the listing removed ( UCEPROTECT ). The pay-to-delist model is not well respected in the email industry. When using blocklist tool checkers, such as MXToolBox, many blocklists will appear, but very few are relevant. Here is our selection of Blocklists you may come across that are least impactful:

Project Honey Pot

ISP Blocklists

Some ISPs use internal blocklists to make blocking decisions. Examples include AOL, Yahoo, Gmail, Outlook and Hotmail. If your IP is being blocked by one of these networks, and those networks have a large presence in your lists, a block of this kind could have a noticeable negative impact on delivery. Marketo monitors for significant ISP blocks. Those experiencing deliverability issues with emails not making it to the Inbox and bulking in the spam folder may benefit from additional services with our Email Delivery Consultants.

Remediation Steps: Email Delivery Compliance Team works to resolve any ISP blocks. ISP blocks are are usually resolved or lifted within less than 24 hours of a delisting request. Customers experiencing significant blocks for Microsoft domains (outlook.com, live.com, microsoft.com), can submit a request to the delivery team to seek mitigation on their behalf.

Additional Resources:

Blocklist Deep Dive

Abuse Report Deep Dive

What is a spamtrap, or spam trap, and why does it matter?

Blocklist remediation

Blocklist resolution flowchart

Successful lead reconfirmation

What is a blocklist?