What causes Spamhaus CSS listings when warming a new domain and IP range?
Matthew Whittaker
Co-founder & CTO, Suped
Published 19 Jul 2025
Updated 19 Aug 2025
6 min read
Warming a new domain and IP range is a critical step in establishing a strong sender reputation. It's designed to gradually build trust with email providers, ensuring your legitimate emails land in the inbox.
However, sometimes, even with the best intentions and a focus on highly engaged subscribers, senders encounter an unexpected hurdle: a listing on the Spamhaus Combined Spam Sources (CSS) blocklist. This can be confusing, especially when you believe you're following best practices.
My experience shows that a CSS listing during warming isn't due to a lack of reputation alone, but rather a reaction to specific sending patterns. It often signals that something has triggered their automated systems, which are designed to detect suspicious activity swiftly. Let's delve into what causes these unwelcome listings and how to navigate them.
The Spamhaus CSS blocklist is an automatically generated dataset that identifies IP addresses involved in sending low-reputation email. It is particularly effective at detecting snowshoe spamming, where spammers try to evade filters by sending small volumes of mail from many different IP addresses.
Unlike some other blocklists (blacklists), CSS listings are not preemptive. They occur in direct response to observed suspicious sending behavior, rather than simply a new or unknown IP address. This is a crucial distinction, as it means a listing is an indicator of an underlying issue, not just a symptom of a new setup.
A common scenario I've seen is when a domain is already listed on the Spamhaus Domain Blocklist (DBL). If an IP address attempts to send mail from a domain that is on the DBL (meaning it has a “used in spam” reputation), those sending IPs are highly likely to also be listed on CSS. The problem isn't the IP range itself, but the associated domain's existing negative reputation.
IP address blocklist reasons
Spam traps: Sending to dormant, invalid, or harvested email addresses. Even a few hits can trigger a CSS listing, especially with a new domain.
Lack of consent: Emails sent to recipients who did not explicitly opt-in to receive them are considered unsolicited and contribute to poor reputation.
Snowshoe spamming: Attempting to distribute low volumes of spam across many IP addresses or domains to evade detection.
Primary triggers for CSS listings
Several factors, especially when combined, can lead to a CSS listing during the warming phase of a new domain and IP range. One of the primary culprits is hitting spam traps. Spam traps are email addresses specifically set up to catch spammers, and they should never receive legitimate mail. Even if your list consists of active openers and clickers, if it inadvertently contains a spam trap, sending to it can immediately flag your new setup as suspicious.
Another significant trigger is the lack of explicit consent from your recipients. Spamhaus (and other anti-spam organizations) primarily focus on unsolicited email. If you're sending to addresses that haven't genuinely opted into your communications, or if your list hygiene isn't pristine, you risk hitting addresses that behave like spam traps or generate complaints, leading to a CSS blocklist (or blacklist).
The combination of a brand new domain and a brand new IP range, particularly when sending similar content to an existing range, can be perceived as snowshoe spamming. While you might be legitimately splitting traffic, Spamhaus's automated systems see this pattern as highly suspicious, especially with new, unestablished sending entities.
The impact of a new domain
The newness of your domain plays a disproportionately large role. Spammers frequently register new domains to bypass filters, so fresh domains are inherently viewed with skepticism until they build a positive reputation. Even if your domain is legitimate, its youth can make any minor misstep, like a single spam trap hit, amplify into a significant reputation blow.
This is why Spamhaus listings are not about complaints, but about spam traps or other indicators of unsolicited mail. While recipients who regularly open and click your emails are highly engaged, this engagement does not prevent spam trap hits if your list acquisition methods are not strictly opt-in. Spam traps do not open emails.
The underlying issue often boils down to list quality and permission. If your list contains addresses that didn't explicitly opt-in, you're always at risk, regardless of how “high quality” your current engaged segments appear. Spamhaus's filters are designed to catch sending that looks like spam, and a new domain sending to even a few bad addresses can instantly fit that profile.
Preventing future CSS listings
To prevent future CSS blocklist (or blacklist) issues during new domain and IP warming, proactive measures are key. Simply warming up a new domain means gradually increasing your sending volume to build a positive sending history.
It is crucial to maintain strict list hygiene. This includes regularly cleaning your lists, removing unengaged subscribers, and ensuring all new subscribers have explicitly opted in. Employing a double opt-in process is the gold standard for list acquisition and significantly reduces the risk of hitting spam traps.
Before using a new domain, especially one that isn't brand new from registration, it's wise to check its history for prior blocklist (or blacklist) issues. A domain with a tainted past can immediately compromise your new IP range, leading to CSS listings even with otherwise good practices.
Finally, monitor your deliverability closely during warming. Pay attention to bounce rates, complaint rates (even if Spamhaus doesn't use them directly), and any other unusual activity. Being proactive in identifying and addressing issues quickly can prevent minor problems from escalating into persistent blocklist (blacklist) challenges.
Best warming practices
Focus on pure opt-in lists with clear consent. Implement double opt-in whenever possible to verify subscriber intent and quality.
Start with your most engaged segments and gradually expand to broader audiences. This builds positive reputation signals.
Regularly clean your email lists to remove inactive subscribers, hard bounces, and any potential spam traps.
Risky warming practices
Sending to purchased, rented, or old, unengaged lists. These often contain spam traps and lead to high complaint rates.
Using a brand new domain for bulk sending without first establishing its reputation, especially in conjunction with new IPs.
Ignoring early signs of trouble like increasing bounce rates or low engagement during warming.
Views from the trenches
Best practices
Always prioritize explicit consent for all subscribers, preferably using double opt-in to verify addresses.
Segment your most engaged subscribers for initial warming efforts to build positive reputation quickly.
Regularly audit your email lists for inactive addresses and potential spam traps to maintain high quality.
Before acquiring a new domain, check its historical reputation to avoid inheriting past issues.
Common pitfalls
Sending to non-opt-in lists or old, uncleaned data, which significantly increases spam trap hit risk.
Attempting to rapidly scale sending volume on a new domain and IP range without careful monitoring.
Ignoring early warning signs like soft bounces or low engagement during the warming process.
Reusing content that has previously generated complaints or poor engagement on other sending IPs.
Expert tips
Implement a feedback loop service to receive spam complaints directly from ISPs, allowing for immediate removals.
Monitor your domain and IP reputation using tools to detect early signs of blocklisting.
Diversify your sending patterns to avoid appearing like a snowshoe spammer.
Understand that
Marketer view
A marketer from Email Geeks says that a CSS listing is not preemptive, it is a direct reaction to something specific that was sent.
2019-11-06 - Email Geeks
Marketer view
A marketer from Email Geeks says that if you are using a new IP range, it is important to understand where it came from.
2019-11-06 - Email Geeks
Key takeaways
While warming a new domain and IP range is essential for email deliverability, encountering a Spamhaus CSS listing highlights the importance of understanding the specific triggers for these blocklists (blacklists). It's rarely about a lack of sending history and almost always about observed problematic behavior.
The combination of a new domain, a new IP, and crucially, sending to unconsented addresses or hitting spam traps, creates a pattern that looks like spamming to automated systems like Spamhaus CSS. By focusing on stringent list hygiene, proper consent, and diligent monitoring, you can effectively navigate the warming process and establish a solid sending reputation for your new infrastructure.
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