Why are IPs blocked by Spamhaus CSS after an infrastructure migration?
Matthew Whittaker
Co-founder & CTO, Suped
Published 4 May 2025
Updated 18 Aug 2025
6 min read
Migrating email infrastructure is a complex undertaking. We recently encountered a situation where hundreds of IPs suddenly appeared on the Spamhaus Combined Spam Sources (CSS) blocklist immediately following a complete infrastructure migration to a new hosting provider. This involved transitioning thousands of IPs to a new Autonomous System Number (ASN) and updating the reverse DNS domains associated with our servers.
The immediate reaction was confusion, as the IPs themselves had a long history of sending. However, the sudden appearance on a major blocklist like CSS indicated that the migration, specifically the change in the ASN and reverse DNS, triggered an automated detection system. This post explores why such a widespread blocklisting can occur after an infrastructure shift and what steps can be taken to resolve and prevent similar issues.
The Spamhaus CSS blocklist is an automatically generated list designed to identify and block IP addresses involved in sending low-reputation email. It’s highly sensitive to sudden changes in sending patterns and infrastructure. When a large volume of email originates from what appears to be a brand-new setup, even if the underlying IPs are old, it can trigger these automated systems.
Automated blocklists (or blacklists) like CSS work by detecting anomalies and patterns associated with spamming. A sudden shift of thousands of IPs to a new ASN, coupled with a new reverse DNS domain, can look suspicious to these algorithms. It mimics patterns often seen with spammers trying to bypass reputation filters by rapidly moving their operations.
The immediate consequence of such a blocklist entry is a significant disruption in email deliverability. Emails sent from the affected IPs will likely be rejected or routed to spam folders by receiving mail servers that consult the Spamhaus CSS list. This can impact critical communications and transactional emails, leading to service interruptions and reputational damage.
Key factors triggering listings after migration
Several factors contribute to IPs being blocklisted by Spamhaus CSS after an infrastructure migration. Even if the IP addresses themselves have a clean history, the context in which they are now operating is entirely new. A new ASN signifies a change in the network routing, which is a key signal for reputation systems. While ASNs are assigned, not created, a recent assignment can raise flags if not handled carefully.
Similarly, a change in the reverse DNS (rDNS) domain can significantly impact reputation. Even if the new rDNS domain is years old, if it has not historically been associated with your high-volume email sending, it will appear as a new sending entity to reputation systems. This lack of established sending history at scale, tied to the new ASN and IPs, looks like a fresh, unvetted operation, which is a major red flag for automated blocklists.
Perhaps the most critical trigger is the absence of a proper IP warmup. When thousands of emails are suddenly sent from a large block of IPs that have no prior sending reputation or volume history in their new environment, it’s akin to a sudden flood of unknown traffic. This abrupt change is exactly what automated systems like Spamhaus CSS are designed to detect and block to prevent spam.
The critical impact of skipping warmup
Skipping IP warmup during a migration can have severe, immediate consequences for your email deliverability. Reputation is built slowly and deliberately, and an overnight switch negates any pre-existing trust. This can lead to widespread rejection of your legitimate emails and persistent blocklistings.
Re-establishing reputation and preventing future issues
To recover from a Spamhaus CSS blocklist and prevent future issues, a structured approach is essential. The most important step for future migrations, or for resolving current issues if possible, is implementing a proper IP warmup strategy. This involves gradually increasing your sending volume from new IPs or infrastructure components, allowing receiving mail servers and blocklists to build a positive reputation over time. This process is crucial to ensure your messages reach the inbox and avoid further blacklisting (or blocklisting) incidents.
Beyond warming, meticulous management of DNS records, especially reverse DNS, is paramount. Ensure your new rDNS configuration accurately reflects your sending identity and that all authentication protocols like SPF, DKIM, and DMARC are correctly set up and aligned. This consistency helps establish trust with mailbox providers and reputation services.
Prior infrastructure
Known ASN: Established network routing with existing reputation.
Established rDNS: Reverse DNS domain had a proven sending history.
Warmed IPs: IP addresses had a built-up, positive sending reputation.
Post-migration challenges
New ASN: The network routing appeared novel and untested.
Unknown rDNS: New reverse DNS domain, even if old, lacked sending history.
Cold IPs: IPs had no established reputation within the new infrastructure.
Sudden volume: Abrupt, high volume from new setup triggered automated systems.
Navigating the delisting process
When facing a Spamhaus CSS blocklist due to migration, contacting Spamhaus directly is often the most effective route. Their FAQs on CSS provide guidance on delisting. Be prepared to explain the circumstances of your migration, including the change in ASN and reverse DNS, and demonstrate that legitimate email is being sent. Automated systems are designed to be cautious, so a human review can often clarify the situation.
While waiting for delisting, consider adjusting your sending practices to minimize further impact. This might involve temporarily pausing non-essential email sends, segmenting your audience, or using an alternative, already warmed infrastructure for critical transactional emails if available. The goal is to reduce the spam signal that the automated systems are detecting.
Remember, a delisting is only a temporary fix if the underlying issues are not addressed. Continuous blocklist monitoring and proactive reputation management are key to long-term email deliverability. This includes ensuring all DNS records are consistent, monitoring your sending metrics, and maintaining good sending practices from your new infrastructure.
Views from the trenches
Best practices
Always plan for a gradual IP warmup when introducing new infrastructure or significant IP changes.
Ensure all relevant DNS records, especially reverse DNS, are correctly configured and reflect your sending identity.
Maintain clear communication with your hosting provider regarding your email sending practices and infrastructure setup.
Common pitfalls
Migrating an entire email sending infrastructure overnight without prior IP warmup or reputation building.
Using a reverse DNS domain that has no prior sending reputation or is associated with a different entity.
Underestimating the impact of sudden, high email volume from a new IP space or ASN.
Expert tips
An ASN is assigned by a Regional Internet Registry, not created, highlighting the importance of understanding the routing environment.
Automated blocklists like Spamhaus CSS react strongly to unusual traffic patterns, such as sudden volume from unestablished IP ranges or ASNs.
Proactively contacting Spamhaus with detailed explanations of your infrastructure changes can aid in the delisting process.
Expert view
Expert from Email Geeks says: A full system block in CSS often indicates a misconfiguration rather than specific spam activity.
2024-05-10 - Email Geeks
Expert view
Expert from Email Geeks says: The combination of a new ASN, new rDNS, and no IP warmup will almost certainly lead to automated CSS listings.
2024-05-10 - Email Geeks
Recovering and building trust
The experience of having IPs blocked by Spamhaus CSS after an infrastructure migration underscores the critical importance of a phased, reputation-aware approach to major email system changes. While the IPs themselves may have been clean, the sudden shift to a new ASN and reverse DNS environment without proper warmup created a pattern that automated blocklists interpreted as suspicious. This highlights how intricate email deliverability can be, extending beyond just content and recipient lists.
Successfully recovering and maintaining a strong sender reputation after such a migration hinges on open communication with blocklist providers like Spamhaus, meticulous attention to DNS configurations, and, crucially, a strategic IP warmup plan. Proactive management and constant monitoring are your best tools for ensuring your legitimate emails consistently reach their intended recipients and avoid future blocklists.
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