How effective are RBLs and blocklists for preventing spam in 2022?

Key findings

• Continued Relevance: RBLs are still viewed as effective tools by some for detecting and preventing spam and phishing, potentially stopping a significant portion (up to 50%) of unwanted traffic.
• Evolving Threats: Spammers constantly adapt, making purely IP-based blocklists less effective against sophisticated attacks that bypass simple content filters.
• False Positives: A significant drawback is the risk of false positives, where legitimate mail is mistakenly blocked, causing deliverability issues.
• Layered Approach: Blocklists are increasingly seen as one component of a broader anti-spam strategy, rather than a standalone solution.
• Sender Utility: For senders, blocklists primarily function as indicators of reputation issues, signaling a need for internal review and remediation.

Key considerations

• Transparency and Criteria: The lack of clear listing and delisting criteria, along with poor communication from some blocklist operators, remains a challenge for senders.
• Business Models: Some blocklists face criticism for perceived unfair practices, such as requiring payment for delisting, which can be seen as a ransom scheme.
• IPv6 Adoption: The rise of IPv6 presents a new frontier for blocklists, with some already listing IPv6 addresses, but broader adoption and effectiveness are still developing. You can learn more about this in a guide to RBLs.
• Sender Best Practices: Maintaining a good sender reputation, avoiding spam, and correctly configuring email authentication protocols like SPF, DKIM, and DMARC are crucial for avoiding blocklists. Read more on DMARC, SPF, and DKIM.

Key opinions

• Transparency Needed: Marketers desire blocklists to provide specific reasons for listings, such as a pristine trap hit or form abuse, to help pinpoint and address problems.
• Actionable Feedback: A key issue is that blocklists often just state an IP or domain is blocked without offering sufficient information for senders to identify and fix the root cause.
• Utility for ESPs: Blocklists are considered valuable for ESPs to find and remove bad actors, especially since spammers frequently change domains and hop between different providers.
• False Positive Risk: Marketers are concerned about false positives, particularly when too many DNSBLs are used with low thresholds, potentially blocking legitimate email.
• Confirmed Opt-In (COI) Debate: Some marketers question why a lack of COI would lead to a major blocklist listing, especially when it results from common issues like typos hitting spam traps.

Key considerations

• Proactive Monitoring: Even with their flaws, marketers understand the necessity of proactive blocklist monitoring to catch issues early and protect sender reputation.
• Cost-Benefit Analysis: While some blocklists offer paid delisting, marketers question the value and ethics of such schemes, seeking more affordable and legitimate solutions.
• Adaptability: Marketers recognize that spammers often blend into the traffic of large ESPs, underscoring the challenge of identification and removal within high-volume environments. This is a crucial aspect of why emails go to spam.
• Comprehensive Approach: For complete spam prevention, marketers should combine blocklist monitoring with robust email authentication and content filtering, as discussed by SpamTitan.

Key opinions

• Professionalism Metrics: The professionalism of a blocklist can be assessed by its listing criteria and the transparency and responsiveness of its operators.
• Sender Responsibility: Many blocklist operators may not fully realize that senders are often the primary users of their feedback for self-correction, not just ISPs for filtering.
• Limited Direct Filtering: IP-based blocklists are generally not very effective at stopping sophisticated spam that isn't easily caught by advanced content filters.
• Incentive for Good Behavior: Blocklists primarily provide leverage to encourage good sending practices among generally well-meaning ESPs and senders.
• Disputed Listing Practices: Some blocklists are criticized for listing policies that penalize IPs for even a single spam trap hit, affecting nearly everyone.

Key considerations

• The Risk of Spite Listings: Open criticism of some blocklists can lead to retaliatory listings, highlighting a concerning lack of accountability in parts of the industry. This mirrors challenges discussed regarding the relevance of certain blocklists.
• Challenges for New Blocklists: Establishing a new and genuinely helpful blocklist in the current environment is seen as highly challenging due to technical complexities and the existing landscape.
• Feedback Loop Design: Operators must balance providing useful feedback with the risk of spammers abusing automated systems to game the blocklisting process.
• Market Dominance: Some very large ESPs are perceived as too big to block, allowing them to sometimes send spam with impunity, which undermines the blocklist system. This problem is also relevant to major mailbox provider deliverability.

Key findings

• Core Function: RBLs and DNSBLs are defined as lists of IP addresses suspected of sending spam, primarily used to prevent unwanted email from reaching inboxes.
• Anti-Spam Efficacy: They are widely considered effective cybersecurity solutions for detecting and preventing spam-like behaviors, including phishing, by blocking email-borne threats.
• Strategic Purpose: Blocklists are often developed to address specific gaps in anti-spam measures or for particular purposes within a network's security framework, as documented by Abusix.
• Mitigation Strategy: Enabling RBLs is a straightforward approach that can stop a significant portion of spam traffic (e.g., up to 50%) at the server level, according to Rackspace documentation.
• Reputation Aggregators: Organizations like Spamhaus and SORBS act as reputation block lists, compiling spam reports from various sources to identify problematic IPs.

Key considerations

• Avoiding Blacklisting: To avoid RBL blacklisting, senders must prioritize not sending spam, correctly configuring security protocols like SPF, DKIM, and DMARC, and actively monitoring their email sending health.
• Limitations: Despite their effectiveness, RBLs have limitations and are often complemented or superseded by more modern spam filtering techniques.
• False Positive Management: While effective, the risk of false positives is a recognized drawback, requiring careful consideration when implementing RBLs into a spam filtering system. Learn more about the impact of DNSBLs on deliverability.
• Evolving Standards: Some blocklists may offer faster and more secure threat reporting via DNS queries compared to older methods like rsync, indicating an evolution in how threat data is disseminated, which is explored further in how email blacklists work.