What are email blacklists, their relative importance, and how do you get listed on them?
Matthew Whittaker
Co-founder & CTO, Suped
Published 15 Jun 2025
Updated 19 Aug 2025
8 min read
Email blacklists, often interchangeably called blocklists, are databases that identify IP addresses or domains with a history of sending unsolicited or malicious emails. They serve as crucial tools for Internet Service Providers (ISPs) and email service providers to filter out unwanted mail, protecting their users from spam, phishing, and other threats. While the concept is straightforward, understanding their relative importance and how to avoid (or get off) them is vital for anyone sending emails, from marketing professionals to transactional email senders.
At their core, an email blacklist (or blocklist) is a compilation of IP addresses and domain names that have been flagged as sources of problematic email. These lists are used by mail servers globally to determine whether to accept or reject incoming email. Getting listed means your emails might not reach their intended recipients, instead being routed to spam folders or blocked entirely.
There are hundreds of blocklists, but they generally fall into two categories: public and private. Public blacklists like Spamhaus are widely used by many organizations and ISPs to filter email. Private blacklists, on the other hand, are maintained by individual email providers (like Gmail or Outlook) for their internal use, based on their own proprietary algorithms and user feedback. You often won't know you're on a private blacklist until your emails start bouncing or landing in spam folders.
Most public blacklists are DNS-based Blackhole Lists (DNSBLs) or Real-time Blackhole Lists (RBLs). These lists contain IP addresses that have been observed sending spam or exhibiting suspicious behavior. When your mail server attempts to send an email to a recipient, the recipient's mail server checks your sending IP address against these DNSBLs. If a match is found, the email may be rejected or heavily filtered. You can delve deeper into what a DNSBL is and how it functions.
Example DNSBL Querybash
dig +short 1.2.3.4.sbl.spamhaus.org
; Check if 4.3.2.1 is listed on Spamhaus SBL
The importance of blacklists for deliverability
The impact of being on a blacklist varies significantly depending on which one it is. Being listed on a major, widely used blacklist like Spamhaus's SBL or URIBL can severely damage your email deliverability, potentially blocking nearly all your outbound mail. Other less influential or hobbyist blocklists might have minimal or no impact, as fewer receivers use them for filtering.
Blacklist Type
Example
Primary Impact
Severity
Major IP/Domain Blocklists
Spamhaus SBL/XBL
Widespread email rejection at major ISPs, affecting most email types.
High
URI Blocklists
URIBL
Emails containing listed URLs may be filtered, even if the sending IP is clean.
Medium
Internal/Private Blocklists
Microsoft/Google
Specific to that provider; emails may go to spam or be rejected for their users.
Varies (can be high for specific targets)
Less Important/Hobbyist Lists
SORBS
Limited or no noticeable impact on deliverability as they are less widely adopted.
Low
The relevance of a blacklist also depends on your sending audience. For B2C senders, Spamhaus is often the most critical. For B2B senders, private blocklists maintained by corporate spam filters like Proofpoint or Barracuda can be more impactful, even if they aren't publicly accessible. It's crucial to understand which blacklists are most important to monitor for your specific email program.
Ultimately, a listing signals poor sender reputation. Email providers use your IP and domain reputation to decide if your emails reach the inbox. A low sender score, often influenced by blacklist appearances, means your messages are more likely to be sent to the spam folder. Knowing how to determine their severity and mitigate issues is essential.
How do you get listed on them?
Getting listed on an email blacklist (or blocklist) primarily happens when your sending practices are perceived as problematic by email receivers. The most common cause is sending unsolicited mail or mail that recipients don't want. This is a broad category that encompasses several specific behaviors.
Spam traps: These are email addresses designed solely to catch spammers. Hitting a spam trap is a strong signal of poor list hygiene or illicit acquisition practices. Pristine spam traps (addresses that have never been valid and are published only to lure spammers) are particularly damaging, often leading to immediate listing on major blocklists like Spamhaus. Recycled spam traps, which are old, abandoned email addresses repurposed as traps, can also lead to listings if your list is not regularly cleaned.
Spam complaints: When recipients mark your email as spam, it directly impacts your sender reputation and can contribute to blacklist listings. High complaint rates tell ISPs that your mail is unwanted.
High bounce rates: Sending to many invalid or non-existent email addresses (hard bounces) indicates a poorly maintained list, which is a common characteristic of spamming operations. This can lead to your IP or domain being flagged.
Sending unsolicited email: This is the fundamental reason. If you're sending to purchased lists, rented lists, or addresses without explicit opt-in, you're at high risk of being blacklisted.
Using email validation tools for illicit purposes: Some tools that scan for active email addresses can appear to blacklists like an attempt to find valid addresses for spamming, even if your intention is to clean your list.
It's not just about what you send, but also how you acquire and manage your lists. Even perfectly legitimate emails can be blocked if they originate from an IP or domain with a history of suspicious activity or association with bad actors. Mail servers also look for specific patterns of network access that are indicative of malicious sending behavior.
Preventing blacklist listings
Preventing a blacklist listing involves adherence to best practices in email marketing and maintaining a strong sender reputation. It's a proactive approach rather than a reactive one.
Good sending practices
Consent: Always use double opt-in for new subscribers to ensure they genuinely want your emails.
List hygiene: Regularly remove inactive subscribers and unengaged addresses to reduce bounce rates and avoid spam traps. This also prevents your emails from going to spam.
Engagement: Send relevant, valuable content that encourages opens and clicks, reducing spam complaints.
Authentication: Implement SPF, DKIM, and DMARC to prove your emails are legitimate and prevent spoofing. Find out more about DMARC, SPF, and DKIM here.
Risks leading to listing
Purchased lists: These lists are notorious for containing spam traps and unengaged users, leading to high complaints and bounces.
Infrequent sending: Long gaps between sends can lead to list decay and increased spam trap hits.
Poor email content: Spammy keywords, excessive images, or broken links can trigger spam filters and user complaints.
Ignoring bounces: Not promptly removing invalid addresses indicates a lack of care for list quality.
Monitoring your sender reputation and checking blocklists regularly are crucial steps. While some blocklist warnings may not significantly impact your deliverability, others require immediate action. Tools are available that can help you check your sender score and proactively address potential issues before they escalate into full blacklistings.
Important for B2B senders
While public blacklists are critical for B2C, B2B email deliverability is often more affected by hidden, internal blocklists managed by large corporations and enterprise-level spam filtering solutions. These lists are not publicly visible, making direct monitoring challenging. Consistent good sending practices and attentive bounce message analysis are your best defense.
Understanding the delisting process
If you do find your IP or domain on a blocklist, the delisting process varies. Major blacklists like Spamhaus typically have clear, automated (or semi-automated) delisting procedures if the cause of the listing has been remediated. Others, particularly some of the lesser-known or less relevant ones, might have more manual, slower, or even non-existent delisting options, which can be frustrating.
The key to successful delisting is to first identify and fix the underlying issue that caused the listing. This could involve cleaning your email list, adjusting your sending volume, improving content, or enhancing email authentication protocols. Only once the root cause is addressed should you attempt to request delisting. Continuously sending problematic mail after delisting will almost certainly result in re-listing.
Being proactive with your email deliverability strategy is far more effective than reacting to a blacklist listing. Regular monitoring of your domain reputation, analyzing bounce messages, and adhering to permission-based sending are fundamental practices that minimize your risk of ending up on a blacklist and ensure your messages are delivered reliably.
Views from the trenches
Best practices
Actively monitor your bounce messages; they often contain clues about blacklist listings and deliverability issues.
Prioritize list hygiene by regularly removing unengaged subscribers and invalid email addresses.
Always obtain explicit consent for sending emails, preferably through a double opt-in process.
Common pitfalls
Assuming all blacklists have the same impact; some are far more critical than others.
Ignoring early warning signs like increased bounce rates or spam complaints.
Using email validation tools that might inadvertently trigger spam trap detection systems.
Expert tips
Focus on the major blocklists like Spamhaus for significant impact, as many others have negligible effect.
For B2B sending, pay close attention to private, internal blacklists of corporate spam filters, which may not be publicly visible.
Understand that legitimate emails can still be blocked if sending practices appear suspicious or if there's an underlying network access pattern issue.
Expert view
Expert from Email Geeks says that if you are listed on Spamhaus or URIBL, you should take immediate action. Otherwise, you should assess the actual impact on your delivery, which is often minimal, and review the listing criteria to see if it is objectively relevant to your operations.
2018-11-18 - Email Geeks
Marketer view
Marketer from Email Geeks says that most blacklists they have encountered appear to be primarily based on spam trap hits. However, they once had a client's office IP address listed due to the use of a local email validation tool that Spamhaus disliked.
2018-11-20 - Email Geeks
Key takeaways for email senders
Email blacklists (or blocklists) are a fundamental part of the internet's defense against spam. While some lists hold more weight than others, understanding their purpose, how you might get listed, and the steps to prevent it is critical for ensuring your emails consistently reach the inbox.
The emphasis should always be on maintaining excellent sending practices, building a clean and engaged subscriber list, and implementing proper email authentication. By doing so, you can significantly reduce your risk of a blacklist listing and maintain a healthy email sender reputation, ensuring your messages are delivered reliably.
Spamhaus
0Spam
Cisco
NoSolicitado
URIBL
abuse.ro
ALPHANET
Anonmails
Ascams
BLOCKEDSERVERS
Calivent Networks
EFnet
JustSpam
Kempt.net
NordSpam
RV-SOFT Technology
Scientific Spam
Spamikaze
SpamRATS
SPFBL
Suomispam
System 5 Hosting
Team Cymru
Validity
www.blocklist.de Fail2Ban-Reporting Service
ZapBL
2stepback.dk
Fayntic Services
ORB UK
technoirc.org
TechTheft
Spamhaus are widely used by many organizations and ISPs to filter email. Private blacklists, on the other hand, are maintained by individual email providers (like 
Gmail or 
Outlook) for their internal use, based on their own proprietary algorithms and user feedback. You often won't know you're on a private blacklist until your emails start bouncing or landing in spam folders.
Microsoft/Google
SORBS
Proofpoint or 
Barracuda can be more impactful, even if they aren't publicly accessible. It's crucial to understand which blacklists are most important to monitor for your specific email program.
