Is SORBS an important email blocklist for deliverability?
Matthew Whittaker
Co-founder & CTO, Suped
Published 6 Jun 2025
Updated 19 Aug 2025
8 min read
When an IP address or domain gets listed on a blocklist (or blacklist, as they're also known), it naturally raises concerns about email deliverability. The Spam and Open-Relay Blocking System (SORBS) is one such widely recognized DNS-based blocklist (DNSBL).
Many internet service providers (ISPs) and mail administrators use various blocklists as a primary defense against unwanted emails, including spam, phishing, and other malicious activities. The question often arises, how important is a SORBS listing compared to other more prominent blocklists, and what does it truly mean for your email program? This guide will delve into what SORBS is, how it operates, its actual influence on email deliverability, and what steps you can take if you find yourself listed.
SORBS, or the Spam and Open-Relay Blocking System, is a DNS-based blocklist that historically focused on identifying and listing IP addresses known for originating spam or being involved in abusive email practices.
This includes servers configured as open relays, which can be exploited by spammers to send unsolicited email messages, or those that host spam traps. The list (or blocklist) compiles data from various sources, including spam traps and monitoring of misconfigured mail servers. If an IP address is found to be involved in sending spam or hosting an open relay, it can be added to the SORBS blacklist.
It's important to understand that while SORBS (the Spam and Open-Relay Blocking System) was historically significant, its primary purpose revolved around identifying servers used for spam, phishing, and other abuses, particularly focusing on open relays. This information can be found on their website, providing context to their operational approach. The email ecosystem has evolved, with many modern mailbox providers using more complex, proprietary algorithms that incorporate a multitude of signals beyond singular blocklist listings.
How SORBS impacts deliverability
When an IP address or domain is listed on any blocklist, including SORBS (the Spam and Open-Relay Blocking System), it can potentially lead to email deliverability issues. This means that emails sent from a listed IP might be rejected, quarantined, or routed directly to the spam folder by receiving mail servers that consult that particular blocklist.
Many large B2C providers, such as Google and Microsoft (formerly Hotmail), rely less on third-party blocklists like SORBS as their primary filtering mechanism. They typically employ their own sophisticated, internal reputation systems that aggregate data from numerous sources, including user feedback, engagement metrics, and various internal spam traps. While SORBS data might be one input among many, a listing here typically doesn't cause a drastic drop in deliverability to these major consumer inboxes.
However, the situation can be different for smaller ISPs, corporate mail servers, and some B2B recipients. These entities might still heavily rely on public DNSBLs (DNS-based blocklists) for their spam filtering. If your recipients are primarily B2B or use smaller, less sophisticated mail systems, a SORBS blacklist (or blocklist) listing could indeed lead to noticeable bounce rates or increased filtering into spam folders. It truly depends on your specific audience and their receiving infrastructure.
B2C mail providers
Major consumer email providers, like Gmail and Yahoo Mail, often have their own comprehensive internal reputation systems. They use a multitude of data points, including user complaints and engagement, to determine inbox placement. While they might consume data from external blocklists (or blacklists) like SORBS as an input, it's rarely the sole determinant of whether an email is delivered or marked as spam. Therefore, a SORBS listing alone might not severely impact deliverability to these large providers.
Impact
Minimal direct impact: Rarely causes widespread deliverability issues.
One of many signals: SORBS data is just one factor in a complex filtering algorithm.
Focus on engagement: User engagement and sender reputation are typically more critical.
B2B mail providers and smaller ISPs
Many business-to-business (B2B) organizations and smaller internet service providers often rely more heavily on third-party DNSBLs for their filtering. These systems might directly block emails from IPs listed on the SORBS blacklist. If your target audience includes a significant number of business domains or smaller email hosts, a SORBS listing could lead to a measurable increase in bounce rates and reduced inbox placement.
Impact
Potential impact: Can lead to higher bounce rates and spam folder delivery.
Direct reliance: Some systems may directly reject based on SORBS listings.
Monitor bounce messages:Check bounce messages to identify specific blocklist rejections.
The direct connection between SORBS and Proofpoint's reputation service is also worth noting. While SORBS itself is described as a non-profit division of Proofpoint, the filtering appliances used by Proofpoint might utilize their own (separate) internal reputation system. This system reportedly has "saner" listing criteria compared to the broader SORBS blocklist, meaning that being listed on SORBS does not automatically guarantee a block by all Proofpoint-secured inboxes. However, the shared spam trap feeds between them suggest that a problematic sending behavior might eventually be picked up by Proofpoint's own filters.
Delisting from SORBS
If your IP address or domain finds itself on the SORBS blacklist (or blocklist), the good news is that delisting is generally considered a low-effort process compared to some other major blocklists. SORBS provides a straightforward delisting mechanism through its website, typically involving a request after resolving the underlying issue that caused the listing. The primary causes for being listed often relate to open relays, compromised servers, or hitting spam traps, so addressing these issues is the first step.
For example, if the listing was due to an open relay, you would need to secure your mail server configuration to prevent unauthorized mail sending. If it was a spam trap hit, reviewing your list hygiene and sending practices to avoid sending to invalid or old addresses would be crucial. Once the root cause is resolved, you can typically submit a delisting request directly on the SORBS website. They may ask for details about the remediation steps taken or provide information on the specific incident that led to the listing.
The process often involves a bit of back-and-forth, where they might provide redacted headers of the message that triggered the listing, helping you pinpoint the source of the issue. While some shared IP addresses might make direct delisting difficult without the hosting provider's cooperation, it is often possible even for shared IPs with the right approach and persistence. It is always advisable to work with your email service provider or hosting company to ensure compliance and prompt delisting.
Checking your IP for SORBS listing
You can check if your IP or domain is listed on the SORBS blocklist (or blacklist) using various online tools. These tools often query multiple DNSBLs simultaneously. While the query method is straightforward, interpreting the results and understanding the specific SORBS sub-list can be important for effective delisting.
Example DNS query for SORBS listingBASH
dig +short 1.2.3.4.dnsbl.sorbs.net A
Replace 1.2.3.4 with your IP address (reversed octets). If you receive an IP address in return (e.g., 127.0.0.x), it indicates a listing. Each .x value corresponds to a specific SORBS sub-list.
Broader context of email blocklists
While a SORBS blacklist (or blocklist) listing might not carry the same weight as being on a list like Spamhaus for major ISPs, it is still crucial to maintain a clean sender reputation across the board. Email deliverability is influenced by a multitude of factors, not just individual blocklist entries.
These include proper email authentication (SPF, DKIM, DMARC), consistent sending volume, low complaint rates, and positive engagement from recipients. Ignoring any blocklist, even those perceived as less impactful, can contribute to an overall negative sender reputation. A single listing might not break your deliverability, but multiple listings across various blocklists (or blacklists), combined with poor sending practices, can significantly hinder your ability to reach the inbox. It is essential to monitor your IP and domain against a comprehensive set of DNSBLs regularly as part of a robust email deliverability strategy.
Best practices to avoid blocklists
Maintain clean lists: Regularly remove invalid or inactive email addresses to minimize spam trap hits.
Implement authentication: Ensure your emails are properly authenticated with SPF, DKIM, and DMARC records.
Secure your servers: Prevent open relays or compromised servers that could lead to listings.
Send relevant content: Avoid sending unsolicited emails and focus on engaged recipients.
Understanding its actual impact
This integrated approach to email health is why understanding the nuances of various blacklists (or blocklists) is essential for anyone sending emails, whether for marketing campaigns or routine business communication. Focusing solely on one or two "major" blocklists while neglecting others can leave blind spots that ultimately affect your email program's success. A good resource for understanding a broader view of blocklists and general email deliverability topics can be found on blogs like Spam Resource.
In summary, while SORBS (Spam and Open-Relay Blocking System) may not be the most impactful blocklist for email deliverability to major B2C providers, it is certainly not irrelevant. Its listings can still affect email delivery, particularly to B2B clients and smaller ISPs that may rely on its data more directly. Therefore, the hosting company's assertion that it's "mostly unused" or "seldom used by any ISP" might be an oversimplification, especially considering its connection to Proofpoint.
My advice is always to address any blocklist listing promptly, even if its perceived impact is low. Delisting from SORBS is generally straightforward and low effort, and resolving the underlying issue that led to the listing is a critical step in maintaining a healthy sender reputation. Proactive monitoring and adherence to email sending best practices are fundamental to ensuring your messages reach their intended recipients, regardless of which specific blocklists (or blacklists) are consulted by various mail systems. Ignoring any blocklist can incrementally chip away at your overall deliverability.
Views from the trenches
Best practices
Always address any blocklist listing promptly, even if the perceived impact seems low, to maintain good sender reputation.
Implement strong email authentication (SPF, DKIM, DMARC) for all outgoing mail, as this is crucial for modern deliverability.
Regularly monitor your IP and domain across various blocklists to catch and resolve issues quickly.
Common pitfalls
Assuming a blocklist is "not important" without understanding your specific recipient base's filtering mechanisms.
Neglecting to fix the root cause of a blocklist listing, which can lead to re-listings or other reputation issues.
Relying solely on external blocklist checks without also monitoring internal bounce messages for rejection reasons.
Expert tips
Check bounce messages for specific rejection reasons, as they often indicate which blocklists (or blacklists) are causing issues.
Even for shared IPs, delisting from SORBS is often possible through persistent communication with the hosting provider or SORBS directly.
If your host charges exorbitant fees for a dedicated IP or doesn't support DKIM, consider evaluating other hosting options for better deliverability infrastructure.
Marketer view
Marketer from Email Geeks says that some business systems do rely on SORBS, and the impact can often be identified by checking bounce messages.
2018-11-17 - Email Geeks
Marketer view
Marketer from Email Geeks says that B2C providers like Gmail do not primarily rely on SORBS, though it might be consumed as one of many inputs.
Spamhaus
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NoSolicitado
URIBL
abuse.ro
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BLOCKEDSERVERS
Calivent Networks
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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
Many large B2C providers, such as Google and 
Microsoft (formerly Hotmail), rely less on third-party blocklists like SORBS as their primary filtering mechanism. They typically employ their own sophisticated, internal reputation systems that aggregate data from numerous sources, including user feedback, engagement metrics, and various internal spam traps. While SORBS data might be one input among many, a listing here typically doesn't cause a drastic drop in deliverability to these major consumer inboxes.
Yahoo Mail, often have their own comprehensive internal reputation systems. They use a multitude of data points, including user complaints and engagement, to determine inbox placement. While they might consume data from external blocklists (or blacklists) like SORBS as an input, it's rarely the sole determinant of whether an email is delivered or marked as spam. Therefore, a SORBS listing alone might not severely impact deliverability to these large providers.
