Why were my dedicated IPs listed on Spamhaus CSS after sending internal test emails?
Matthew Whittaker
Co-founder & CTO, Suped
Published 8 May 2025
Updated 17 Aug 2025
5 min read
It can be incredibly frustrating to discover your dedicated IP addresses listed on Spamhaus CSS (Combined Spam Sources) shortly after you've begun sending. Even more so when those initial emails are just internal tests to your own team, and the IPs have a supposedly clean history. This scenario raises immediate questions: could it be a false positive, or is there a deeper issue at play?
The Combined Spam Sources (CSS) blocklist is highly automated and reacts very quickly to perceived spam activity. It's designed to list IP addresses that have been detected sending mail matching Spamhaus's heuristics. Unlike some other blocklists, CSS listings are often short-lived, typically expiring three days after the last detection.
While a listing on CSS after only a few internal test emails might seem like an anomaly, it usually points to an underlying issue that needs to be addressed. We'll explore the common reasons this happens and what steps you can take to prevent recurring blocklistings (or blacklistings).
Spamhaus maintains several blocklists, and CSS is a component of their comprehensive ZEN blocklist. It's designed for speed and responsiveness, quickly identifying and listing IP addresses that exhibit spam-like characteristics. This means that even a small volume of email can trigger a listing if the patterns match known spam heuristics.
While sending to internal team members might seem safe, it's possible for test emails to inadvertently trigger CSS if certain conditions are met. For example, if the recipient addresses include old or disused email accounts that have been converted into spam traps, or if the email content itself resembles common spam, a listing can occur. This is particularly true for newly warmed IPs where reputation is still being established.
Spamhaus’s stance on listings
Spamhaus generally does not issue false positive listings without cause. As they state in their CSS FAQs, an IP is listed because it was detected sending mail matching their heuristics. Even if you believe the mail is legitimate, their systems have identified something that triggered the block. It’s crucial to understand their primary goal: to protect internet users from unwanted email, not to serve senders.
The hidden client reputation problem
When dedicated IPs get listed quickly, especially with a new client, it's often a strong indicator that the client's historical sending practices or current data quality are the root cause. A new client might be bringing a poor reputation with them, even if they're now using fresh IPs.
Relying solely on a client's word or a simple questionnaire about consent isn't always sufficient. Bad list acquisition methods, such as single opt-in, purchased lists, or poorly maintained databases, can introduce high levels of risk. These lists are prone to containing spam traps, invalid addresses, and disengaged recipients who are more likely to mark emails as spam, leading to blocklistings (or blacklistings).
Insufficient vetting
Many organizations simply rely on client self-reporting regarding list acquisition and consent, which often leads to hidden issues. This approach can be a significant risk factor for email deliverability.
Questionnaires: Solely trusting client-filled questionnaires about consent without further verification.
Implicit trust: Assuming a reputable client (e.g., a bank) automatically means clean sending practices.
No verification: Failing to audit how email addresses are collected or validated.
Robust vetting
Effective client vetting involves proactive steps to verify email list quality and sending practices, which is crucial for maintaining a healthy sending reputation.
Consent verification: Demanding proof of explicit, verifiable consent for all subscribers, preferably double opt-in.
List audit: Requiring access to and auditing of email acquisition sources and list hygiene practices.
Reputation history: Investigating past sending reputation and any previous blocklist appearances for the client's domains.
Even with small internal test sends, if the client has a history of problematic behavior, Spamhaus might link it to the new infrastructure. This can happen through various signals, including domain association or patterns in email content, making it seem like the new IPs are simply a continuation of prior issues.
Technical checks and list hygiene
While human error can occur, it's essential to meticulously review your sending infrastructure. Check that all authentication records, including SPF, DKIM, DMARC, and PTR records, are correctly configured. Misconfigurations can sometimes trigger passive DNS listings, even without active mail flow.
Even if your client claims to have consent, a lack of rigorous list hygiene is a frequent culprit. Typo domains, particularly those collected through offline or online registration forms without verification, are prime candidates for becoming spam traps. Sending to these addresses, even accidentally, will immediately trigger blocklists like CSS. Proactive list validation before sending any emails is a critical step.
Trigger
Description
Prevention
Spam traps
Emails sent to addresses specifically designed to identify spam activity. Can be active or recycled.
Implement double opt-in. Regularly clean lists. Validate new addresses.
Poor list hygiene
Sending to old, inactive, or unengaged subscribers, increasing bounces and spam complaints.
Segment lists by engagement. Re-engage or remove inactive contacts.
Client reputation
A new client's past poor sending practices or problematic domains that affect new IPs.
Conduct rigorous due diligence. Require transparent sending practices.
Content issues
Emails containing known spam phrases, suspicious links, or malicious attachments.
Spamhaus can list IPs based on DNS records if misconfigured or associated with known bad actors.
Ensure all DNS records (including PTR, HELO) are correctly configured.
Views from the trenches
Best practices
Implement double opt-in for all new email address acquisitions.
Conduct thorough due diligence on all new clients and their sending history.
Regularly audit and clean your email lists to remove inactive or problematic addresses.
Ensure all DNS records (SPF, DKIM, DMARC, PTR) are properly configured.
Common pitfalls
Solely relying on client's self-reported consent without independent verification.
Ignoring signs of past poor sending practices from a new client.
Failing to validate email lists before beginning new sending campaigns.
Assuming low sending volume protects from blocklist listings.
Expert tips
Spamhaus's CSS is automated, and a listing means their systems detected something.
Listings can occur due to passive DNS detection, not just active email sending.
Spamhaus prioritizes protecting recipients from unwanted mail over sender convenience.
If a client repeatedly triggers blocklists, they are likely the root of the problem.
Expert view
Expert from Email Geeks says a new client might carry poor reputation, leading to immediate blocklisting.
August 9, 2023 - Email Geeks
Marketer view
Marketer from Email Geeks says they have accidentally been assigned dirty IPs from their ESP on new projects.
August 17, 2023 - Email Geeks
Navigating blocklist challenges
Having your dedicated IPs listed on Spamhaus CSS, even after sending only a handful of internal test emails, highlights the nuanced and often challenging world of email deliverability. It's a clear signal that something triggered an automated system, and while the volume was low, the perceived threat was high.
The primary takeaway is that a CSS listing is rarely a random event or a simple false positive. It often points to deeper issues related to the client's historical reputation, their list quality, or subtle technical misconfigurations that might be picked up through passive DNS monitoring. Addressing these root causes is paramount, especially when dealing with Spamhaus, whose focus is firmly on protecting recipients.
Moving forward, prioritize rigorous client vetting, enforce stringent list hygiene practices like double opt-in, and continuously monitor your sending infrastructure. Remember, simply getting delisted without resolving the underlying problem will likely lead to recurring blocklistings (or blacklistings). Maintaining a clean sender reputation requires proactive and diligent effort from the outset.
Spamhaus
0Spam
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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
Spamhaus maintains several blocklists, and CSS is a component of their comprehensive ZEN blocklist. It's designed for speed and responsiveness, quickly identifying and listing IP addresses that exhibit spam-like characteristics. This means that even a small volume of email can trigger a listing if the patterns match known spam heuristics.
