How should ESPs warm up a large number of new IPs on shared pools while avoiding Spamhaus listings?
Matthew Whittaker
Co-founder & CTO, Suped
Published 7 Aug 2025
Updated 16 Aug 2025
9 min read
Warming up new IP addresses is a critical phase for any Email Service Provider (ESP), especially when dealing with a large number of IPs within shared pools. The goal is to establish a strong sender reputation with Internet Service Providers (ISPs) and mailbox providers, ensuring your emails reliably reach the inbox. However, this process becomes significantly more complex when you're introducing dozens of new IPs simultaneously, particularly due to the risk of being listed on blocklists, such as Spamhaus Combined Spam Sources (CSS). I've seen many ESPs struggle with this, inadvertently triggering blocklists due to missteps in their warm-up strategy. The key is to understand that IP warm-up isn't just about sending volume, but about demonstrating legitimate sending behavior.
The traditional advice for IP warm-up often focuses on gradual volume increases, but this can be challenging when you have a high demand for new IP capacity. The immediate need for new customer sending coupled with the need for cautious, reputation-building practices creates a unique tension. When you factor in the automated and reputation-sensitive nature of major blocklists, the stakes are even higher. I often emphasize that an effective warm-up strategy for large IP pools needs to be meticulously planned and executed.
My experience shows that rushing this process or using suboptimal practices can lead to severe deliverability issues, including broad blockages from major mailbox providers like Microsoft Apple, and Gmail. It is crucial to approach this with a strategy that balances the need for scale with the imperative of building trust with receivers.
Understanding the challenge of large-scale IP warm-up
One of the primary reasons ESPs encounter blocklisting when warming a large number of IPs is due to practices that resemble snowshoeing. This refers to sending low volumes of mail across a very large number of IP addresses in an attempt to evade reputation filters. While it might seem counterintuitive, sending too few emails across too many IPs can be flagged as suspicious behavior. For instance, distributing 500 emails across 50 new IPs means only 10 emails per IP, which is an extremely low volume that can raise red flags for blocklist operators like Spamhaus.
Spamhaus CSS listings often indicate either suspect behavior, misconfiguration, or a poor sending reputation. When new IPs are introduced, they lack established trust. If the initial email streams, however small, hit spam traps, the ratio of bad to good sends on those nascent IPs can be alarmingly high, leading to rapid blocklisting. This is why the quality of the list used for warm-up is paramount.
Traditional IP warm-up
Focus: Gradually increasing email volume from a single IP or a small set of IPs.
Challenge: Slow for large-scale IP additions. Can appear as snowshoeing if applied incorrectly across many IPs.
Misconfigurations also play a significant role. If DNS records (like rDNS, SPF, DKIM, and DMARC) for the new IPs are not properly set up, it can trigger automated blocklists. These technical details are often overlooked in the rush to scale, but they are fundamental to establishing a trustworthy sending identity. I always recommend a thorough audit of all DNS records before commencing any warm-up.
Strategic approaches to warming large IP pools
For ESPs needing to warm up a large number of IPs, a more sophisticated approach is required than a simple linear volume increase. The core idea is to break down the daunting task into manageable, reputation-building segments. Instead of spreading a single mail stream across all new IPs, I advocate for grouping IPs and associating them with distinct sender profiles or domains.
For example, if you have 50 new IPs, consider breaking them into 10 groups of 5. Each group should have a dedicated sending domain and be warmed up with separate, high-quality customer traffic. This approach mitigates the risk of snowshoeing because each group establishes its own distinct reputation. This ensures that even if one group faces a challenge, it doesn't immediately compromise the entire pool. This strategy also aligns with the advice for warming up an IP address for email sending more broadly.
Critical to this strategy is using only real, engaged traffic. Warm-up is not a magic trick, but a process of introducing your sending patterns to mailbox providers in a way that builds trust. This means directing new customers or segments of highly engaged existing customers to the new IP groups for their initial sends. Remember, each mailbox provider (Gmail, Yahoo, Microsoft) evaluates reputation independently. Sending good mail to one doesn't automatically boost your standing with another. A well-executed warm-up ensures how ESPs manage IP pools positively impacts deliverability.
Traditional warm-up
Single IP or small IP set, gradually increasing volume over time. The focus is on a slow ramp-up without much segmentation.
Reputation is built on one or a few IPs, which can be slow and risky if any issues arise, as they impact the entire sending stream from those IPs.
Traffic: Often mixed, making it harder to pinpoint list quality issues during warm-up.
Avoiding blocklists (or blacklists) like Spamhaus is paramount during the IP warm-up phase. Mailbox providers widely respect and utilize these lists to filter incoming mail. A listing, even a short-lived one, can severely hamper your deliverability. The critical takeaway is that blocklists react to poor behavior. While spam traps might detect issues, the underlying cause is always the sending practices themselves.
For ESPs, this means rigorously validating customer lists before allowing them to send on new IPs. Even a small number of spam traps or inactive addresses can disproportionately impact a low-volume, warming IP. It's not just about what's in your list, but what's not: unengaged recipients, purchased lists, or old data are recipes for disaster. I've always stressed that how email blacklists actually work is tied directly to sender behavior.
Key strategies to avoid blocklists
List quality: Use only highly engaged, permission-based lists for initial warm-up sends. Avoid any questionable sources.
Segmentation: If warming many IPs, group them and warm each group with separate, high-quality traffic streams.
Configuration: Ensure all DNS records (rDNS, SPF, DKIM, DMARC) are correctly configured for every new IP. A simple email deliverability test can help.
In addition to proper setup and segmentation, ESPs must ensure that new IPs receive a diverse yet controlled flow of email traffic. Diversifying the recipient ISPs (e.g., sending to Google, Microsoft, and Yahoo proportionally) helps build a holistic reputation. However, never try to warm up for one ISP by sending volume to another; this is ineffective. Each receiver evaluates your sending independently, so direct engagement with each major mailbox provider is necessary. A good understanding of blocklists is key.
The role of data and continuous monitoring
Successful IP warm-up, especially for large volumes, relies heavily on continuous monitoring and data analysis. It's not enough to set a schedule and hope for the best. You need to actively track key metrics to ensure your IPs are building positive reputation. This means paying close attention to bounce rates, complaint rates, and engagement metrics (opens and clicks) for each warming IP group. Early detection of anomalies can prevent minor issues from escalating into major blocklistings.
Utilize available postmaster tools from major providers like Google Microsoft and Yahoo. These tools provide invaluable insights into your sending reputation, spam rates, and delivery errors directly from the receiving end. They can show you if your new IPs are hitting spam traps or experiencing other issues specific to their networks, which is crucial for making real-time adjustments. This goes hand-in-hand with general technical solutions for email deliverability.
Ultimately, the speed at which you can warm up a large number of IPs is dictated by the quality of your traffic and your ability to monitor and react to feedback. There's no shortcut for building a solid reputation. A dedicated IP warm-up process, even when scaled, requires patience and strict adherence to best practices. This ensures that new IPs integrate smoothly into your shared pools without negatively impacting your overall deliverability, and helps prevent emails from going to spam.
Final thoughts on scaling warm-up
Metric
Why it matters
Target during warm-up
Spam complaint rate
Indicates how many recipients mark your emails as spam. High rates severely damage reputation.
Below 0.1% (ideally close to 0%)
Bounce rate
Reflects the percentage of emails that couldn't be delivered. High rates suggest poor list hygiene.
Below 2%
Open rate
Measures recipient engagement. Higher rates signal positive reception and good sender reputation.
As high as possible, typically above 15-20% for marketing emails.
Click-through rate (CTR)
Indicates deeper engagement. A strong CTR reinforces positive sender reputation.
Consistent with historical engaged traffic, typically above 2-3%.
Unsubscribe rate
While not directly a negative, sudden spikes can indicate content or frequency issues.
Below 0.5%
The successful warm-up of a large number of new IPs on shared pools hinges on a strategic, quality-driven approach. It means moving beyond a simple volume-based schedule and embracing robust list hygiene, segmented sending, and proactive monitoring. By understanding that IP reputation is built on consistent, legitimate sending behavior and diligent attention to deliverability metrics, ESPs can navigate the complexities of large-scale IP warm-up while effectively avoiding detrimental blocklist listings like those from Spamhaus.
Ultimately, the speed at which you can warm up a large number of IPs is dictated by the quality of your traffic and your ability to monitor and react to feedback. There's no shortcut for building a solid reputation. A dedicated IP warm-up process, even when scaled, requires patience and strict adherence to best practices. This ensures that new IPs integrate smoothly into your shared pools without negatively impacting your overall deliverability, and helps prevent emails from going to spam.
Views from the trenches
Best practices
Validate all customer lists rigorously before using them for new IP warm-up to prevent spam trap hits.
Segment large groups of new IPs into smaller, manageable sub-pools for independent warming with distinct sender profiles.
Ensure all DNS records (rDNS, SPF, DKIM, DMARC) are perfectly configured for every new IP.
Monitor postmaster tools from major ISPs for reputation feedback specific to each warming IP or group.
Common pitfalls
Spreading very low email volumes across a large number of new IPs, which can be flagged as snowshoeing behavior.
Assuming that good deliverability on existing IPs means new IPs will perform similarly without proper warm-up.
Ignoring early warning signs from postmaster tools or blocklist notifications, leading to widespread issues.
Relying on outdated or unengaged email lists for warm-up, significantly increasing the risk of spam traps.
Expert tips
Snowshoeing is a real risk. If you are warming a large number of IPs, group them into smaller clusters, each with a unique sending domain and email identity.
Spamhaus CSS listings are rarely mistakes; they usually indicate problematic sending behavior or misconfiguration, even with low volumes.
Warm-up is about demonstrating consistent, good sending behavior with real, engaged traffic, not just increasing volume.
A high spam trap hit rate on a small volume of emails can be more detrimental than a few hits on a large, established volume.
Marketer view
Marketer from Email Geeks says that mailbox providers are not transparent about the specific blocklists they use for filtering decisions.
2023-06-23 - Email Geeks
Expert view
Expert from Email Geeks advises against warming too many IPs simultaneously and notes that Spamhaus rarely errs, often indicating spamtrap hits from poor sending behavior.
Microsoft
Apple, and 
Gmail. It is crucial to approach this with a strategy that balances the need for scale with the imperative of building trust with receivers.
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
Google Microsoft
and Yahoo. These tools provide invaluable insights into your sending reputation, spam rates, and delivery errors directly from the receiving end. They can show you if your new IPs are hitting spam traps or experiencing other issues specific to their networks, which is crucial for making real-time adjustments. This goes hand-in-hand with general technical solutions for email deliverability.
