Which mailbox providers use Spamhaus listings to block email and how does it affect deliverability?
Michael Ko
Co-founder & CEO, Suped
Published 27 May 2025
Updated 16 Aug 2025
6 min read
Spamhaus is one of the most significant and widely respected organizations in the fight against email spam and abuse. Their blocklists (or blacklists) are used globally to help filter out unwanted messages, playing a crucial role in maintaining clean inboxes for users everywhere. Being listed on a Spamhaus blocklist can have a dramatic effect on your email deliverability.
The challenge for senders, however, is that mailbox providers (MBPs) rarely disclose the exact combination of anti-spam technologies and data feeds they use. This includes their specific reliance on third-party blocklists like Spamhaus. Each provider has its own proprietary 'secret sauce' for filtering incoming email, designed to protect their users and adapt to evolving spam tactics.
Spamhaus manages several key blocklists, each targeting different types of spam activity. These lists are essentially real-time databases that identify IP addresses and domains associated with spamming, phishing, malware, or other malicious activities. They are primarily published as DNS-based Blocklists (DNSBLs) or Real-time Blackhole Lists (RBLs), making them easily queryable by mail servers.
Their most impactful lists include the Spamhaus Block List (SBL), which lists IP addresses with a poor reputation, and the Domain Block List (DBL), which focuses on domain names. The Combined Spam Sources (CSS) list identifies IP addresses involved in sending low-reputation email. All of these are integrated into the Spamhaus ZEN zone, a comprehensive single query source.
It is important to remember that these lists do not directly block emails, but rather provide data that mailbox providers use as a component in their own spam filtering decisions. When your email server attempts to deliver mail, the receiving server may query Spamhaus's DNSBLs. If your IP address or domain is found on a blocklist, the receiving server can then choose to reject the email, mark it as spam, or apply further scrutiny.
Mailbox providers leveraging Spamhaus data
While a definitive, exhaustive list of mailbox providers (MBPs) using Spamhaus listings is not publicly available due to their proprietary nature, many major players in the email ecosystem are known or strongly suspected to integrate Spamhaus data into their filtering mechanisms. This makes staying off these lists critically important for any sender.
Providers like Yahoo, Microsoft (for Outlook and Exchange Online), Comcast, and Apple (especially for DBL data) have been observed to use or have previously used Spamhaus blocklists as part of their inbound filtering processes. While Gmail does not officially state direct reliance, it is widely believed that they factor Spamhaus data into their complex sender reputation algorithms, indirectly affecting deliverability.
This indirect usage means that even if a mailbox provider doesn't strictly block emails based on a Spamhaus listing, being on a blacklist can significantly contribute to a sender's overall negative reputation score with that provider. A lower reputation can lead to emails being throttled, delayed, or placed in the spam folder, even without an explicit block message.
Mailbox provider
Known or suspected Spamhaus usage
Impact on deliverability
Yahoo
Historically used, likely continues to use SBL/DBL data.
Significant impact. Listing can lead to rejections or spam folder delivery.
Microsoft (Outlook, Exchange Online)
Known to use Spamhaus for inbound filtering.
High impact. Listings frequently result in email blocks and spam placement.
Gmail
Does not officially confirm direct use, but likely factors listings into reputation scores.
Indirect impact. Can lead to higher spam rates or throttling.
Comcast / RoadRunner / Sky
Known to use Spamhaus listings for filtering.
Direct impact. Listings often result in email rejections.
Apple (iCloud Mail)
Believed to use Spamhaus DBL data for filtering.
Likely impact, especially for domain-related issues.
The profound impact on email deliverability
The impact of a Spamhaus listing on your email deliverability can be profound. When an IP address or domain is actively listed on one of their primary blocklists, such as the SBL or DBL, emails originating from that source are highly likely to be rejected or routed directly to the spam folder by receiving mail servers that consult these lists.
This can lead to a significant and immediate drop in your email delivery rates, impacting critical communications such as transactional emails, marketing campaigns, and customer service messages. For businesses, this translates directly into missed opportunities, reduced engagement, and potential damage to brand reputation.
Understanding the different types of listings, particularly between informational and active (or actionable) listings, is crucial for senders. Each carries a different level of immediate threat to your inbox placement and overall email program. You can also explore what happens when your IP is blocklisted for more detailed information.
Distinguishing informational and active Spamhaus listings
Spamhaus differentiates between various types of listings. Active listings (such as those on the SBL, DBL, or CSS) indicate that an IP or domain is actively involved in spamming or poses a significant threat, prompting mail servers to block or heavily filter emails from those sources. These require immediate attention and remediation.
Informational listings, on the other hand, serve more as a warning. They highlight potential issues or concerning behavior that could lead to an active listing if not addressed. While they generally do not cause immediate email blocks, they indicate that your sending practices are being monitored and could impact your overall sender reputation. You can read more about changes to the Spamhaus DBL for domain-specific insights.
Active listings (SBL, DBL, CSS)
Nature: Indicates current, verifiable spamming or abusive activity.
Impact: Leads to immediate email rejections or direct delivery to spam folders.
Action required: Urgent investigation and remediation, followed by a delisting request.
Informational listings
Nature: Warns of suspicious or problematic sending behavior, not necessarily active spamming.
Impact: Typically does not cause direct blocking, but can influence reputation scores.
Action required: Proactive review of sending practices to prevent escalation to active listings.
Proactive strategies to avoid blocklists
Given Spamhaus's widespread influence, implementing proactive strategies to avoid being listed is paramount for maintaining optimal email deliverability. This begins with robust list hygiene, ensuring you are only sending to engaged, permission-based contacts. Regularly cleaning your email lists helps eliminate inactive or invalid addresses that could turn into spam traps, which are a common cause of blocklist entries.
Beyond list quality, strong email authentication is crucial. Properly configuring SPF, DKIM, and DMARC records helps mailbox providers verify that your emails are legitimate and from an authorized sender. This significantly reduces the likelihood of your emails being flagged as spam or outright rejected. You can consult an in-depth guide to email blocklists for more comprehensive insights.
Key practices to maintain a clean sender reputation
Obtain explicit consent: Always use double opt-in processes for new subscribers.
Monitor engagement: Regularly remove inactive or unengaged subscribers from your lists.
Implement authentication: Ensure SPF, DKIM, and DMARC are correctly configured for your sending domains.
Watch for spikes: Keep an eye on your bounce rates and spam complaint rates.
Address issues quickly: If you receive a warning or listing, act immediately to remediate the cause.
While knowing the exact list of mailbox providers that directly use Spamhaus listings remains elusive, its undeniable influence across the email ecosystem means that a listing on any of their primary blocklists (whether an IP blacklist or a domain blocklist) should always be treated with urgency.
Whether through direct blocking or indirect influence on sender reputation scores, Spamhaus data significantly shapes whether your emails reach the inbox. Prioritizing email health through adherence to best practices, continuous monitoring, and rapid response to issues is essential for maintaining a strong sender reputation and ensuring your messages consistently reach their intended recipients.
Views from the trenches
Best practices
Actively monitor your IP and domain reputation across major blocklists like Spamhaus. Early detection is key.
Segment your audience and send relevant content to engaged subscribers to maintain high engagement rates.
Regularly clean your email lists to remove invalid, inactive, or role-based addresses that can become spam traps.
Implement and maintain all standard email authentication protocols (SPF, DKIM, DMARC) for all sending domains.
Ensure proper feedback loop (FBL) setup with major mailbox providers to receive and act on spam complaints.
Common pitfalls
Ignoring informational listings, assuming they have no impact, can lead to more severe active blocklists later.
Sending to old, unengaged, or purchased lists, which are often riddled with spam traps and invalid addresses.
Failing to monitor bounce rates and spam complaint rates, missing early warning signs of deliverability issues.
Not having a clear process for delisting if you do end up on a Spamhaus blocklist, causing prolonged disruption.
Overlooking domain reputation, focusing solely on IP reputation, as many blocklists target domains now.
Expert tips
If your IP is listed on Spamhaus, investigate the root cause immediately, whether it's compromised accounts or poor list hygiene.
For Yahoo and Gmail, while direct Spamhaus use is debated, focusing on high engagement and low complaints is always the best strategy.
Consider transactional and marketing email streams separately, as their sending patterns and risks differ.
Educate internal teams about email best practices to prevent inadvertent actions that could lead to blocklists.
Don't solely rely on bounce messages for blocklist alerts; implement dedicated blocklist monitoring.
Marketer view
Marketer from Email Geeks says many mailbox providers use Spamhaus listings, but a comprehensive, public list is unlikely due to their confidential filtering methods.
2022-08-23 - Email Geeks
Expert view
Expert from Email Geeks says they previously compiled a list of ISPs using Spamhaus from client bounces, noting that Yahoo, Comcast, and possibly Microsoft used it, and Gmail likely consumes some Spamhaus reputation data indirectly.
Spamhaus
0Spam
Cisco
NoSolicitado
URIBL
abuse.ro
ALPHANET
Anonmails
Ascams
BLOCKEDSERVERS
Calivent Networks
EFnet
JustSpam
Kempt.net
NordSpam
RV-SOFT Technology
Scientific Spam
Spamikaze
SpamRATS
SPFBL
Suomispam
System 5 Hosting
Team Cymru
Validity
www.blocklist.de Fail2Ban-Reporting Service
ZapBL
2stepback.dk
Fayntic Services
ORB UK
technoirc.org
TechTheft
Yahoo, 
Microsoft (for Outlook and Exchange Online), 
Comcast, and 
Apple (especially for DBL data) have been observed to use or have previously used Spamhaus blocklists as part of their inbound filtering processes. While 
Gmail does not officially state direct reliance, it is widely believed that they factor Spamhaus data into their complex sender reputation algorithms, indirectly affecting deliverability.
