Which mailbox providers use Spamhaus listings to block email and how does it affect deliverability?

Key findings

• Widespread adoption: Many mailbox providers, including significant players like Yahoo and Microsoft, are known to integrate Spamhaus data into their filtering processes.
• Indirect impact on gmail: While Gmail does not explicitly state its use of Spamhaus, experts suspect it factors such listings into its reputation scoring, indirectly affecting deliverability.
• Impact on deliverability: A Spamhaus listing, particularly on core blocklists like the SBL (Spamhaus Block List) or XBL (Exploits Block List), can result in significant email blocking, sometimes affecting a large percentage of email traffic.
• Informational listings: While less severe than active blocks, informational listings (like those on the Spamhaus CSS, or Composite Blocking List), can still raise flags and prompt further scrutiny from mailbox providers.
• Proactive approach: Many email service providers (ESPs) and companies (like Constant Contact) proactively use Spamhaus data to prevent sending from listed IPs or domains.

Key considerations

• Assume widespread use: It's prudent for senders to assume that most major mailbox providers leverage Spamhaus blocklists (or blacklists) in some capacity, making avoidance of these listings a critical deliverability goal. To ensure proper deliverability, learn how email blocklists work.
• Monitor actively: Regularly monitor your IP addresses and domains for Spamhaus listings (and other important blocklists) to address issues promptly.
• Remediation focus: If listed, prioritize understanding the cause and taking immediate steps for delisting to mitigate deliverability impact.
• Reputation data: Understand that even if a mailbox provider doesn't directly block based on a specific Spamhaus list, the data likely contributes to your sender reputation, affecting inbox placement.

Key opinions

• Lack of transparency: Marketers acknowledge that mailbox providers keep their filtering methods confidential, making it difficult to compile a definitive, up-to-date list of who uses Spamhaus.
• Observed impact: Despite the lack of official confirmation, marketers frequently observe deliverability issues to major providers like Yahoo, Comcast, and Microsoft when their IPs or domains are listed on Spamhaus.
• Indirect gmail involvement: While some claim Gmail doesn't use Spamhaus, many marketers suspect it indirectly influences Gmail's reputation scores.
• DBL influence: Apple is believed to use Spamhaus DBL (Domain Block List) data for their filtering.
• Informational listings concern: A recent spike in Spamhaus informational listings has led to increased client inquiries and concern among marketers, even though these listings typically do not result in direct blocks.

Key considerations

• Proactive avoidance: It is crucial for marketers to prioritize staying off Spamhaus lists entirely, as a listing, regardless of the explicit MBP usage, will likely cause deliverability headaches. Learn more in our guide to Spamhaus blacklisting.
• Client education: Marketers often need solid data to convince clients about the serious impact of Spamhaus listings, especially given the rise of less impactful informational listings.
• Comprehensive monitoring: Beyond Spamhaus, marketers should monitor other significant blocklists to maintain optimal deliverability and ensure their emails reach the inbox effectively. Explore different types of email blocklists.
• Treat all listings seriously: Even if an informational listing doesn't immediately block mail, it signals potential issues that could escalate if not addressed.

Key opinions

• Confidential filtering: Experts agree that mailbox providers keep their spam filtering logic, including specific blocklist usage, confidential, making it impossible to obtain an official list.
• Probable usage: Based on observed bounce patterns and industry knowledge, experts confirm that Yahoo, Comcast, and Microsoft are highly likely to use Spamhaus listings.
• Gmail's indirect approach: Experts largely suspect that Gmail, while not directly blocking based on Spamhaus, incorporates its data into its overall reputation scoring, leading to indirect deliverability impacts.
• Impact on apple: It is strongly believed that Apple utilizes Spamhaus DBL data as part of its email filtering processes.
• Severity of all listings: Regardless of whether a listing is active or informational, experts stress that any Spamhaus listing is detrimental to sender reputation and overall deliverability.

Key considerations

• Universal vigilance: Experts recommend always striving to avoid any Spamhaus listing, as the potential negative impact on email deliverability is significant across the board. Understand what happens when your IP gets blocklisted.
• Reputation is key: Focus on maintaining a strong sender reputation, as blocklist data contributes to overall reputation scores used by mailbox providers, even if not for direct blocking. Improve your domain reputation.
• Data-driven decisions: Utilize bounce data and deliverability monitoring to identify if Spamhaus listings are causing specific blocks, which helps in targeted remediation efforts.
• Educate stakeholders: Be prepared to explain the nuances of Spamhaus listings, including the difference between informational and active blocks, to clients and internal teams to manage expectations effectively.

Key findings

• Core function: Spamhaus provides real-time databases of IP addresses and domains identified as spam sources or threats, designed to aid in email security and filtering.
• Zen combined feed: The ZEN blocklist is a combination of several Spamhaus IP-based blocklists (SBL, SBLCSS, XBL, PBL), offering a single, powerful feed for filtering.
• PBL usage: The Policy Block List (PBL) identifies IP addresses that should not be sending direct-to-internet email, typically dynamic or non-MTA IPs, helping to enforce email policies.
• DBL for domains: The Domain Block List (DBL) lists domains found in spam messages, impacting email where the domain appears in the From, Reply-To, or URL headers.
• Informational listings (CSS): The CSS (Composite Blocking List) can include IP addresses with poor sending practices, even if not actively spamming, serving as an early warning.

Key considerations

• Understand listing criteria: Familiarize yourself with the specific criteria for each Spamhaus list, as different lists target different types of problematic behavior (e.g., spam, exploits, policy violations). This knowledge is crucial for fixing Spamhaus CSS listings.
• Importance of delisting: Documentation provides clear instructions for requesting removal from Spamhaus lists once the underlying issues have been resolved. Timely action is essential to restore deliverability. Learn how to get delisted from Spamhaus ZEN.
• Proactive best practices: Adhering to email sending best practices, maintaining clean mailing lists, and ensuring proper email authentication (SPF, DKIM, DMARC) are vital for avoiding future listings.
• Consequences for bad actors: Spamhaus documentation emphasizes its role in identifying and tracking spammers, which helps justify its strict listing policies to the broader internet community.