Which email blocklists are most important for deliverability and how should they be prioritized?

Summary

A comprehensive approach to email deliverability necessitates understanding and strategically prioritizing blocklists. The importance of any particular blocklist depends significantly on the recipient's email provider and geographical location. Proactive monitoring of major blocklists like Spamhaus, Barracuda, and Proofpoint is crucial, but niche or regional lists should also be considered based on the target audience. Experts emphasize that domain-based lists are useful for identifying problematic customer content, while IP-based lists often signal severe issues. Some blocklists are not publicly available, requiring direct observation of blocked mail. Documentation from various sources highlights the specific purposes and functions of different blocklists, such as the Spamhaus SBL/DBL, Barracuda BRBL, Proofpoint DRF, and URIBL. Furthermore, it's crucial to view blocklists as diagnostic tools rather than definitive judgments, prompting investigations into underlying issues like list hygiene and account security. Relying solely on blocklists is insufficient; a holistic approach that includes sender reputation, engagement metrics, and email authentication methods (SPF, DKIM, DMARC) is essential for sustained deliverability.

Key findings

  • Recipient Relevance: The importance of blocklists is highly dependent on which email providers your recipients use and their geographic location.
  • Proactive Monitoring of Major Lists: Proactive monitoring of Spamhaus, Barracuda, and Proofpoint is crucial due to their widespread impact.
  • Domain vs. IP Importance: Domain-based lists identify problematic customer content, while IP-based lists signal severe issues.
  • Diagnostic Use of Lists: Blocklists should be used as diagnostic tools to uncover underlying problems like list hygiene and security issues.
  • Comprehensive Deliverability Strategy: A comprehensive deliverability strategy requires more than just blocklist monitoring. Sender reputation, authentication, and engagement metrics are all vital.
  • Impact of Specific Lists: Spamhaus listings directly damage delivery. SURBL listings correlate with poor email practices.
  • Non-Public Lists: Some blocklists are not publicly available and require observation of mail being blocked.

Key considerations

  • Tailor Monitoring to Audience: Tailor your blocklist monitoring efforts to the specific demographics and email providers used by your target audience.
  • Investigate Underlying Issues: A blocklist listing should trigger an immediate investigation into potential list hygiene problems, compromised accounts, or other problematic sending practices.
  • Prioritize Sender Reputation: Focus on improving and maintaining a positive sender reputation by following best practices for list management, engagement, and content creation.
  • Implement Authentication: Ensure proper implementation of SPF, DKIM, and DMARC to improve email authentication and demonstrate legitimacy to mailbox providers.
  • Monitor Domain and IP Reputation: Actively monitor both your domain and IP reputation to identify and address any potential issues that could lead to blocklisting.
  • Use Multiple Monitoring Tools: Combine various tools and services to get a more comprehensive view of blocklisting status.

What email marketers say
11Marketer opinions

The importance and prioritization of email blocklists for deliverability depend on several factors, including the recipient's email provider, target audience, and the specific goals of the sender. Major blocklists like Spamhaus, Barracuda, and Proofpoint have a broad impact and should be proactively monitored. However, smaller or regional blocklists can be crucial for specific audiences. A comprehensive deliverability strategy involves monitoring blocklists, maintaining a good sender reputation, and employing email authentication methods.

Key opinions

  • Blocklist Impact: The impact of a blocklist varies depending on the recipient's email provider. Major blocklists have a wider reach, while niche lists affect specific regions or industries.
  • Monitoring Priority: Prioritize monitoring blocklists relevant to your target audience and those widely used by major mailbox providers.
  • Proactive Monitoring: Proactively monitor key blocklists such as Spamhaus, Barracuda, and Proofpoint to identify and address deliverability issues early.
  • Comprehensive Approach: Blocklist checks are part of a broader deliverability strategy that includes sender reputation, authentication, and engagement metrics.
  • Multiple Tools: Using multiple tools and services for monitoring provides a more comprehensive view of your blocklist status.

Key considerations

  • Regional Relevance: Consider the geographical location of your target audience when prioritizing blocklist monitoring.
  • List Hygiene: Maintain clean email lists and sender reputation to avoid being listed on blocklists.
  • Authentication: Implement email authentication methods (SPF, DKIM, DMARC) to demonstrate legitimacy and improve deliverability.
  • Sender Reputation: Actively work to improve sender reputation and engagement metrics as these are critical to avoid blocklisting.
  • Diagnostic Tool: Use blocklists as a diagnostic tool to identify underlying issues such as list hygiene problems or compromised accounts.
Marketer view

Email marketer from Email Marketing Forum shares to prioritize blocklist monitoring based on your target audience. If you primarily send emails to users in Europe, focus on lists that are commonly used by European ISPs and mailbox providers. Tailor your monitoring efforts to the regions and industries you serve.

July 2023 - Email Marketing Forum
Marketer view

Email marketer from Litmus says blocklists are like a giant blacklist for IP addresses and domains that have been associated with spam. If you land on one, your emails might not make it to the inbox.

February 2022 - Litmus
Marketer view

Email marketer from Reddit suggests focusing on major blocklists like Spamhaus, Barracuda, and URIBL. These lists are widely used and can significantly impact your deliverability. Don't waste time on obscure lists with little impact.

September 2024 - Reddit
Marketer view

Marketer from Email Geeks explains that the importance of any RBL depends on how many of your recipients' mailbox providers implement it. For instance, being listed on Nosolicitado is harmful if your recipients are in South America, but not if they are in Europe.

May 2023 - Email Geeks
Marketer view

Email marketer from SendPulse Blog shares that the impact of a blocklist depends on the recipient's email provider. Major blocklists like Spamhaus and Barracuda Central have a broad impact, while smaller, niche lists might affect deliverability to specific regions or industries. Prioritize monitoring lists that are relevant to your target audience.

August 2022 - SendPulse Blog
Marketer view

Email marketer from Deliverability Blog shares that using blocklist checks as part of a broader deliverability strategy is important. While blocklists are important, also consider sender reputation, authentication, and engagement metrics to ensure consistent deliverability.

March 2021 - Deliverability Blog
Marketer view

Email marketer from GlockApps Blog suggests proactively monitoring key blocklists such as Spamhaus, Barracuda, and Proofpoint. These lists are widely used by mailbox providers and security vendors to filter spam. Regularly checking your IP address and domain against these lists can help identify and address deliverability issues before they escalate.

December 2024 - GlockApps Blog
Marketer view

Email marketer from ZeroBounce details maintaining clean lists, monitoring sender reputation, and authenticating your email (SPF, DKIM, DMARC) are key to avoiding blocklists. These practices show you’re a legitimate sender.

February 2025 - ZeroBounce
Marketer view

Email marketer from Mailjet Blog emphasizes the importance of monitoring blocklists to maintain a good sender reputation. Being listed on a blocklist can significantly impact deliverability, leading to emails being marked as spam or blocked entirely. Monitoring helps identify and resolve issues quickly.

November 2024 - Mailjet Blog
Marketer view

Marketer from Email Geeks shares if a blocklist affects deliverability for more than a few recipients, it's a priority. Lists like RFC-ignorant, may warrant contacting the MX operator to explain the high probability of false positives. Proactive monitoring should include Spamhaus, Barracuda and Proofpoint.

July 2023 - Email Geeks
Marketer view

Email marketer from StackExchange suggests using multiple tools and services to monitor blocklists. No single tool covers every list, so using a combination provides a more comprehensive view of your blocklist status.

November 2024 - StackExchange

What the experts say
7Expert opinions

Experts emphasize that blocklists impact email deliverability differently and should be approached strategically. Spamhaus listings directly damage delivery, while SURBL listings correlate with poor practices. Domain-based lists are crucial for identifying problematic customer content, whereas IP-based listings indicate severe issues. Some blocklists are not public and require direct observation of blocked mail. Meaningful querying of DBL/URIBL requires access to email content, making it more relevant for ESPs. Blocklists are diagnostic tools, not definitive judgments, highlighting underlying problems. Relying solely on blocklists is insufficient; a comprehensive view including engagement and sender reputation is necessary.

Key opinions

  • Spamhaus Impact: Being listed on Spamhaus directly damages email delivery.
  • SURBL Correlation: SURBL listings correlate with poor email practices.
  • Domain vs. IP Lists: Domain-based lists help identify bad customer content, while IP-based lists signal severe issues.
  • Non-Public Lists: Some blocklists are private, requiring observation of blocked mail for detection.
  • DBL/URIBL Querying: Meaningful querying of DBL/URIBL requires email content access, more useful for ESPs.
  • Diagnostic Use: Blocklists are diagnostic tools, revealing underlying issues, not definitive judgments.
  • Comprehensive View: Deliverability needs a comprehensive approach beyond blocklists, including engagement and sender reputation.

Key considerations

  • Customer Investigation: Domain-based hits necessitate investigating customer content.
  • Behavioral Monitoring: Dedicated IP listings indicate a need for improved behavioral monitoring.
  • Underlying Issues: Blocklist listings should prompt investigation into list hygiene and account security.
  • Engagement & Reputation: Prioritize building strong sender reputation and positive engagement metrics.
  • Strategic Use: Use blocklists as part of a wider strategic view for deliverability not as the only element.
Expert view

Expert from Email Geeks shares to meaningfully query the DBL (or URIBL), you need access to the content of mail being sent, which is more useful for ESPs.

November 2024 - Email Geeks
Expert view

Expert from Word to the Wise shares solely focusing on blocklists is not enough for deliverability. A broader perspective, including engagement and sender reputation, is necessary to achieve consistent inbox placement, as some lists are poorly managed.

August 2022 - Word to the Wise
Expert view

Expert from Email Geeks shares domain-based lists are more interesting because a customer's content triggering one indicates a need to investigate the customer. IP-based hits are a sign that a customer is bad, after all other signs have been missed.

April 2023 - Email Geeks
Expert view

Expert from Email Geeks explains some blocklists are not publicly available and you can only know you're on them if mail is actually blocked.

August 2023 - Email Geeks
Expert view

Expert from Spam Resource explains blocklists should be used as a diagnostic tool, not a definitive judgment of sending practices. Being listed can indicate underlying issues that need attention, such as list hygiene problems or compromised accounts.

January 2023 - Spam Resource
Expert view

Expert from Email Geeks notes that dedicated IP listings may indicate the customer is sending mail to recipients who didn't ask for it, pointing to problematic behavior. Reputation providers may then analyze content (hostnames) to identify similar bad content elsewhere.

August 2023 - Email Geeks
Expert view

Expert from Email Geeks says being listed on Spamhaus will directly damage delivery, while being listed on SURBL correlates with poor practices that lead to poor delivery.

June 2023 - Email Geeks

What the documentation says
5Technical articles

The documentation highlights several important blocklists and sender support resources that impact email deliverability. Spamhaus maintains lists like SBL (IP addresses of spam sources) and DBL (domains found in spam), each serving a distinct purpose. Barracuda Central's BRBL is a real-time database of IPs with poor reputations, used by Barracuda Networks for spam filtering. Proofpoint's DRF analyzes email traffic to identify and block spam based on sender reputation, while Microsoft provides sender support resources for deliverability to Outlook.com and Hotmail users. URIBL categorizes URLs found in spam, flagging malicious domains and phishing attempts. Understanding these lists and resources is essential for troubleshooting and improving email sending practices.

Key findings

  • Spamhaus Lists: Spamhaus maintains distinct blocklists like SBL (IPs) and DBL (domains) for identifying spam sources.
  • Barracuda BRBL: Barracuda's BRBL identifies IPs with poor reputations used for spam filtering.
  • Proofpoint DRF: Proofpoint's DRF analyzes email traffic and sender reputation to block spam.
  • Microsoft Sender Support: Microsoft provides resources for senders to ensure deliverability to Outlook.com and Hotmail users.
  • URIBL Categories: URIBL categorizes URLs found in spam, flagging malicious domains and phishing attempts.

Key considerations

  • List Differentiation: Understand the specific purpose of each blocklist for effective troubleshooting.
  • Reputation Improvement: Listing on BRBL indicates the need for improved sending practices.
  • Traffic Analysis: Recognize that Proofpoint analyzes traffic and sender reputation to identify spam.
  • Microsoft Policies: Adhere to Microsoft's policies and best practices for optimal delivery to their users.
  • Content Scrutiny: Be aware that URIBL flags URLs in spam for malicious content and phishing attempts.
Technical article

Documentation from URIBL explains their lists categorize URLs found in spam. Some lists focus on new or potentially malicious domains, while others flag URLs associated with phishing or malware. Understanding these categories is useful for identifying compromised content.

February 2025 - URIBL
Technical article

Documentation from Barracuda Central details the Barracuda Reputation Block List (BRBL) is a real-time database of IP addresses with poor reputations. It is used by Barracuda Networks' email security products to identify and block spam. If you find your IP address on the BRBL, it indicates that your sending practices need improvement.

December 2022 - Barracuda Central
Technical article

Documentation from Microsoft discusses their sender support space. If you are having trouble sending emails to Microsoft users, their documentation provides information on their policies and best practices for ensuring deliverability. This is crucial for any sender targeting Outlook.com or Hotmail users.

September 2022 - Microsoft
Technical article

Documentation from Spamhaus explains Spamhaus maintains several blocklists, each with a specific purpose. The SBL (Spamhaus Block List) lists IP addresses of known spam sources, while the DBL (Domain Block List) lists domains found in spam. Understanding the differences between these lists is crucial for effective troubleshooting and delisting.

September 2022 - Spamhaus
Technical article

Documentation from Proofpoint describes the Proofpoint Dynamic Reputation Filter (DRF) uses real-time analysis of email traffic to identify and block spam. It assesses sender reputation based on various factors, including sending volume, content, and engagement metrics. Being listed on the DRF can significantly impact deliverability to organizations using Proofpoint's email security solutions.

January 2022 - Proofpoint