What are the most important email blacklists to monitor and how do you check them?

Key findings

• Spamhaus dominance: Spamhaus remains the most universally respected and widely used blocklist, with its PBL, SBL, and XBL (formerly CBL) lists having significant impact on deliverability globally. Understanding how to get listed on Spamhaus is crucial.
• SURBL importance: SURBL (Spam URI Realtime Blocklists) is highly influential, particularly for URL-based listings, and should be a top priority for monitoring. It detects spam by checking message bodies for URIs (links) that appear in spam.
• Barracuda relevancy: Barracuda Central's blacklist (b.barracudacentral.org) is also considered significant, especially for B2B senders, as many corporate networks use Barracuda spam filters.
• Invaluement status: Invaluement's SIP, SIP24, and URI lists remain operational and impactful, though their querying method (requiring a license key) means they are primarily used by subscribers and not widely accessible to public checkers. Public tools that claim to check Invaluement might be using outdated hostnames.
• SpamCop and Lashback: SpamCop (bl.spamcop.net) still holds some weight, particularly in B2B environments or specific regions like Australia due to its integration with filters like Mimecast. However, Lashback's unsubscore.com (UBL) is generally regarded as less reliable or impactful these days.

Key considerations

• Prioritize monitoring: Focus on the blacklists that demonstrably impact your email volume and deliverability. Not every blocklist carries the same weight, and some have minimal or zero impact on modern email delivery.
• Leverage logs: Analyze your email logs and bounce messages to identify which specific blocklists are causing delivery failures. This empirical approach provides the most accurate picture of what is truly affecting your sending.
• Automated monitoring: Implement a system for automated blacklist checking to receive timely notifications if your IP addresses or domains get listed. This proactive approach allows for quick remediation.
• Understand listing criteria: Familiarize yourself with the listing criteria of the most important blacklists. This knowledge helps prevent future listings by identifying and correcting poor sending practices. More information can be found on sites like Campaign Monitor's Email Blacklists 101.
• Customer feedback: Beyond monitoring tools, pay close attention to customer complaints about emails not being received. This feedback can sometimes be the earliest indicator of a deliverability issue, including blocklist listings.

Key opinions

• Spamhaus and SURBL are top concerns: Marketers consistently express the most concern over listings on Spamhaus (SBL, PBL, XBL) and SURBL, recognizing their significant influence on email delivery across various mailbox providers.
• SpamCop has niche relevance: While not as universally impactful as Spamhaus, SpamCop still affects deliverability for specific audiences, particularly those sending B2B emails or into regions like Australia, where Mimecast spam filters utilize it.
• Irrelevant lists can be distracting: There's a strong sentiment that receiving notifications from obscure or low-impact blacklists can cause unnecessary alarm and divert resources from more pressing deliverability issues. The focus should be on blocklists that genuinely affect email volume.
• Data-driven decisions: The best way to determine a blacklist's importance is by analyzing delivery logs and traffic patterns to see which blocklists are causing bounces or rejections for specific email campaigns. This helps prioritize monitoring efforts effectively.
• Invaluement's unique access: Some marketers acknowledge Invaluement's continued relevance, but note its reliance on license keys for access means it's not a public check and requires a direct relationship or integration with a provider that has a license.

Key considerations

• Focus on impact, not just listing: Don't panic over every listing. Instead, evaluate the actual impact on your email deliverability. A listing on a less influential blacklist might not warrant immediate action.
• Monitor B2B specific blacklists: If your primary sending is B2B, pay closer attention to blacklists like Barracuda and SpamCop, as these are often utilized by corporate email filters.
• Tailor your monitoring: Customize your blacklist monitoring based on your specific sending profile and target audience. For a more comprehensive understanding of these lists, consult Mailgun's guide to blocklists.
• Prioritize high-volume blacklists: As outlined in our guide on which blacklists are most important, direct your resources towards blocklists that directly result in email rejection or spam folder placement at major ISPs.
• Ongoing reputation management: Blacklist monitoring is a part of broader sender reputation management. Continuous adherence to best practices, such as proper list hygiene and consent, is vital for long-term deliverability.

Key opinions

• Multiple dimensions of relevance: An expert from Email Geeks points out that a blocklist's relevance can be judged by whether mail is directly blocked, if it's used as a data source for other decisions, or if a listing correlates with poor delivery or poor sender practices. This multi-faceted view is crucial.
• CBL/XBL clarification: Experts confirm that CBL (cbl.abuseat.org) is defunct under that name and has long been integrated into Spamhaus's XBL. While some systems still query the old hostname, the data comes from Spamhaus XBL.
• Lashback's diminished standing: There's a consensus among experts regarding the lack of respect for Lashback's processes, suggesting its unsubscore.com (UBL) list should not be a primary concern for senders.
• Value in minor lists: Even less impactful lists, such as Spam Eating Monkey, can serve a purpose by indicating underlying poor sending practices, even if they don't directly block mail at major ISPs. Getting listed often means something is wrong with your email acquisition or sending habits.
• Invaluement's private nature: An expert from Invaluement clarifies that their SIP, SIP24, and URI lists use license-key-based hostnames, making direct public queries impossible. This means external checkers using old hostnames will provide inaccurate results, potentially listing everything due to no response or an errant 127.0.0.2 response.

Key considerations

• Beyond direct blocking: Recognize that blocklists can influence deliverability not only by direct blocking but also by contributing to a sender's overall reputation score used by ISPs. For a detailed exploration, see our in-depth guide to email blocklists.
• Distinguish public vs. private: Understand the difference between public (freely queryable) and private (subscription-based or internal) blacklists, as this affects how you monitor them and interpret results. Our page on public vs private blacklists provides more insight.
• Monitor evolving relevance: The relevance of blocklists changes over time. Regularly re-evaluate which lists are most impactful based on industry trends and your own delivery data.
• Address underlying issues: A listing, even on a less critical blocklist, often signals underlying issues with your email program, such as poor list acquisition practices, spam trap hits, or high complaint rates. Addressing these root causes is paramount.
• Beware of biased listings: Some blocklists may employ overly broad or even arbitrary listing practices (e.g., UCE Protect's broad listings or personal vendettas), which can make delisting difficult and their data less reliable for objective deliverability assessment.

Key findings

• Purpose of RBLs: RBLs, or DNSBLs, are real-time databases of IP addresses or domains known to send spam. Mail servers query these lists to make decisions on incoming email, rejecting or flagging messages from listed entities. This system is a primary defense against unsolicited mail.
• Listing criteria: Common reasons for listing include sending to spam traps, high complaint rates, unsolicited bulk email (UBE), and operating open relays or compromised servers. Specific lists may have unique criteria.
• Delisting procedures: Most blacklists provide a web interface for checking listings and requesting delisting. The process typically requires the sender to demonstrate that the cause of the listing has been remediated and that they have implemented safeguards against future issues.
• Automated listing: Many listings are automated based on various triggers, such as spam trap hits, honeypots, or excessive spam reports. This automation necessitates a prompt and automated monitoring response from senders.
• Impact on deliverability: Being on an important blacklist can lead to severe deliverability problems, including emails being rejected outright or routed directly to spam folders, regardless of content quality.

Key considerations

• Consult official sources: Always refer to the official documentation of each blacklist for the most accurate and up-to-date information regarding their policies and delisting procedures. This is more reliable than third-party interpretations.
• Implement best practices: Adhere to email sending best practices, including maintaining a clean mailing list, obtaining explicit consent, and providing easy unsubscribe options, to avoid listing on any blocklist.
• Understand DNSBL queries: Familiarize yourself with how mail servers query DNSBLs, as detailed in RFCs and technical guides. This understanding helps in diagnosing deliverability issues. For more, see our guide on what is a DNSBL.
• Proactive monitoring: Use automated monitoring solutions that frequently check the most influential blacklists. Early detection allows for faster remediation, minimizing potential impact on your email campaigns.
• Severity of domain listings: Understand that a domain listing can be particularly damaging as it affects all email originating from that domain. Our page on what happens when your domain is blacklisted outlines these consequences.