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

Key findings

• Audience specificity: The impact of a blocklist listing is directly tied to whether your recipients' mailbox providers use that specific list. A list critical in one geographic region may be irrelevant in another.
• Public vs. private lists: Highly impactful blocklists, such as those used by Cloudmark or Vade, are often private and cannot be publicly queried. You may only discover a listing when your mail is actively blocked. This contrasts with publicly available lists like Spamhaus.
• Nature of listing: There's a crucial distinction between IP-based and domain-based blocklists. Domain-based listings (affecting the sending domain or content domains) can indicate issues with specific customer content, offering an opportunity to catch problems early. IP-based listings often signal more pervasive sending issues, indicating broader problems with sender behavior. More details are available in our guide on how email blacklists actually work.
• Proactive monitoring: Relying solely on public blocklist monitoring is insufficient. It is essential to monitor sending behavior, engagement metrics, and feedback loop data to detect issues before they result in blocklistings.

Key considerations

• Prioritize based on impact: Any blocklist listing that significantly impacts deliverability for a notable portion of your recipients should be a priority. This pragmatic approach helps focus resources where they matter most.
• Understand listing nuances: For lists like RFC-ignorant or other insignificant email blocklists, direct engagement with the MX operator using the list to highlight its false positive rate may be more effective than attempting to delist.
• Integrate data sources: Combine insights from blocklists with data from sending behavior, complaint rates, and other tools like Google Postmaster Tools and SNDS to form a comprehensive deliverability strategy. More on this topic can be found in a Mailgun article discussing best practices to improve email deliverability.
• Address underlying issues: A blocklist listing is often a symptom of deeper problems, such as poor list hygiene or sending unwanted mail. Focus on rectifying these root causes rather than just seeking immediate delisting.

Key opinions

• Prioritization is key: Marketers frequently question the existence of a definitive list of most important email blacklists, suggesting that impact is fluid and context-dependent rather than fixed.
• Impact varies by recipient: A listing's relevance is often determined by whether a statistically significant number of target recipients use a mailbox provider that references that specific Real-time Blackhole List (RBL).
• Beyond public visibility: Some blocklists, particularly those run by large appliance vendors (like Cloudmark and Vade), don't offer public query options, making direct monitoring difficult. Marketers often only become aware of these listings when mail is actively blocked.
• Domain vs. IP: There's an ongoing discussion about whether domain-based or IP-based listings are more indicative of deliverability issues, with some marketers emphasizing the importance of domain reputation, particularly when using DKIM for client domains.

Key considerations

• Geographic relevance: Consider the geographical distribution of your audience when assessing a blocklist's importance. A list highly impactful in one region might be negligible in another.
• Proactive vs. reactive: The ideal approach is proactive, integrating blocklist monitoring with other data sources like sending behavior and API integrations to identify potential issues early. Waiting until a public listing occurs is often too late, as discussed in this guide on why your emails are going to spam.
• Data quality: Marketers frequently encounter issues with old or poor-quality subscriber data, which can lead to inadvertent blocklist hits. Regular list hygiene is paramount.
• Focus on root causes: Instead of merely seeking delisting, investigate the underlying reasons for being blocklisted, such as sending to unengaged recipients or compromised accounts. This is a core part of effective email deliverability strategies, as highlighted in this article on the ultimate guide to mastering email deliverability.

Key opinions

• Spamhaus's unique role: Spamhaus is often cited as the exception among blocklists due to its direct and significant impact on delivery across many mailbox providers. A listing here warrants immediate attention.
• Behavioral correlation: Lists like SURBL (Spam URI Realtime Blocklists) correlate with poor sending practices, meaning a listing indicates underlying behavioral issues that will independently lead to poor deliverability.
• The problem with public lists: Many of the most impactful blocklists, beyond Spamhaus, are not publicly queryable. This means senders may only learn of a listing through mail rejections, reinforcing the need for internal monitoring and a robust deliverability strategy.
• Differentiating IP and domain hits: Domain-based blocklist hits often serve as an early warning for problematic customer content, allowing for intervention before significant damage occurs. IP-based hits, conversely, are typically the last sign that a customer is sending problematic mail, implying that earlier warning signs were missed.

Key considerations

• Focus on customer behavior: For IP listings, the core issue is often customer behavior, such as sending to recipients who did not opt-in (sometimes referred to as spam traps). Reputation providers then analyze content from these streams to identify similar bad content from other sources.
• Historical context matters: Be wary of blocklists that don't visibly block mail in your logs, or those with highly opinionated or non-standard delisting processes (like requiring Usenet posts). These are often less impactful. You can learn more about this in our brief history of email blacklists and blocklists.
• Differentiation in reporting: IP-based and domain-based blocklist hits should be treated and reported on differently. Domain hits are proactive warnings, while IP hits are reactive indicators of deeper behavioral problems.
• Big players and spam: Even major email services like Google, Microsoft, and Sendgrid can be significant sources of spam, leading some smaller (and often ineffective) blocklists to list them. While understandable from a frustration perspective, such listings are generally pointless as they do not affect overall deliverability for those large providers. More general info on this can be found in a guide to the impact of email blacklists.

Key findings

• Standard for abuse handling: RFCs like RFC 2142 establish standard addresses for network operations, including abuse@ for reporting network abuse. Compliance with such RFCs is a basic expectation for senders, and failure can lead to reputation issues.
• Mailbox provider requirements: Major mailbox providers often publish their own sender guidelines, which implicitly or explicitly dictate how they use blocklists and what behaviors will trigger them. These are crucial for understanding deliverability to specific destinations.
• Best practices for reputation: Documentation consistently emphasizes the importance of good sending habits, proper authentication (SPF, DKIM, DMARC), and managing subscriber engagement to prevent issues that lead to blocklistings.
• Automated listing processes: Many blocklists operate on automated or algorithmic triggers based on spam trap hits, high complaint rates, or unusual sending patterns, as outlined in technical guides on what spam traps are and how they work.

Key considerations

• Adherence to RFCs: Ensuring your organization adheres to Internet Engineering Task Force (IETF) RFCs, such as RFC 2142, by properly managing abuse complaints, contributes to a positive overall reputation and reduces the likelihood of being flagged by any blocklist that values compliance. Here is the direct link to the IETF RFC 2142 documentation.
• Implement authentication: Strong email authentication (SPF, DKIM, DMARC) is a fundamental requirement outlined in numerous documentation sources. While not directly a blocklist, failing authentication can lead to reputation damage that makes blocklist listings more likely.
• Monitor delivery logs: Even for private blocklists, bounce messages and rejection logs often contain clues (e.g., specific error codes or blocklist names) that indicate a listing. Analyzing these logs is paramount for identifying issues quickly.
• Maintain list hygiene: Regularly cleaning email lists to remove inactive or problematic addresses, including known spam traps, aligns with documentation best practices for avoiding blocklistings.