How to warm up email sending after migrating to a new ESP with the same domain and shared IPs?
Matthew Whittaker
Co-founder & CTO, Suped
Published 10 Aug 2025
Updated 18 Aug 2025
7 min read
Migrating to a new Email Service Provider (ESP) is a significant undertaking, and it often raises questions about the necessity of email warm-up, especially when retaining your established sending domain and using shared IP addresses. It’s a common misconception that if your domain has a good reputation and you're moving to shared IPs (which are theoretically already warmed by the ESP), you can bypass the warm-up process. However, this is rarely the case.
Email deliverability is a complex ecosystem, and while domain reputation plays a crucial role, it's not the only factor. Internet Service Providers (ISPs) and mailbox providers (MBPs) evaluate a combination of signals to determine trustworthiness, including the sending IP address, the domain, and the consistency of sending patterns. A sudden change in any of these components, even if the domain remains the same, can trigger spam filters.
Therefore, a strategic warm-up is almost always recommended. This process allows you to gradually introduce your sending patterns and domain-IP combination to the new environment, building a positive sending relationship with mailbox providers on the new infrastructure. It's about demonstrating consistent, desired sending behavior from the new ESP's shared IP pool.
Why warm-up is still necessary
When you migrate to a new ESP, even if you keep your existing sending domain, you are essentially moving to new underlying infrastructure. The new ESP will have its own set of shared IP addresses, and while these IPs might be generally 'warmed' by other senders, your specific domain's traffic has not yet been associated with them by mailbox providers. Each combination of sending domain and IP address builds its own unique reputation over time.
Mailbox providers maintain detailed profiles for sender reputation, which include both domain and IP address components. A sudden influx of emails from a familiar domain but on unfamiliar IPs (even shared ones) can look suspicious, potentially leading to increased spam folder placement or even blocklisting. This is why a smooth, controlled transition is crucial to maintain, or even improve, your email deliverability. For more on this, you can read about why warm-up is communication.
Therefore, the warm-up process isn't just for new domains or dedicated IPs. It's about introducing your established domain's sending patterns to a new set of IP addresses and mail servers. This helps mailbox providers like Google and Yahoo to recognize and trust your email volume coming from the new ESP's infrastructure. It minimizes the risk of your emails landing in the spam folder or encountering other email deliverability issues.
Domain reputation
Your domain's reputation is built over time by consistent sending practices, low complaint rates, and high engagement from recipients. This reputation is tied to your domain name itself, making it portable between ESPs if managed correctly. However, a sudden shift in the origin of your email traffic, even with a strong domain, can be perceived as unusual by ISPs, especially if the new IP addresses are unfamiliar to your domain's sending history.
Shared IP reputation
Shared IPs carry a collective reputation from all senders using them. While a good ESP maintains clean shared IPs, your specific domain hasn't established its own relationship with these particular IPs yet. Mailbox providers observe the combination of your domain and the new IP addresses. A warm-up period allows your domain to gradually build trust with this new IP pool, integrating your sending patterns into the existing reputation of the shared IPs without causing disruptions.
The mechanics of shared IP warm-up
The core principle of warming up, whether you're using dedicated or shared IPs, is to start with a small volume of highly engaged recipients and gradually increase it. This tells mailbox providers that your emails are valued by recipients, reinforcing a positive sender reputation. For high-volume senders, this process might seem lengthy, but it's a critical investment in your long-term deliverability.
Your ESP should provide a recommended warm-up schedule. Generally, you start with a modest daily volume (e.g., 3,000 to 5,000 emails per day to start, even for very large lists) and increase it incrementally, typically by 20-25% each day or every few days, depending on performance. The key is to monitor your engagement rates, bounce rates, and spam complaint rates closely. If you see negative trends, slow down the increase or even reduce volume until metrics recover. You can find a comprehensive guide on IP warming on Spam Resource.
During this period, focus your sending on your most active and engaged subscribers first. These are the contacts most likely to open, click, and not mark your emails as spam, which sends strong positive signals to mailbox providers. As your volume increases, you can expand to less active segments, always prioritizing quality and engagement over raw volume. This strategic approach helps solidify a positive reputation for your domain on the new ESP's shared infrastructure, making the warm-up period more effective.
Day
Volume Sent
Engagement Focus
Day 1-3
3,000-5,000
Most engaged subscribers
Day 4-6
5,000-10,000
Highly engaged subscribers
Day 7-10
10,000-20,000+
Engaged segments
Ongoing
Gradual increase to full volume
Monitor engagement and complaint rates closely
Key considerations and challenges
While shared IPs theoretically absorb some of the warm-up burden, you must still monitor your performance diligently. Watch for signs of deliverability issues, such as increased bounce rates, higher spam complaints, or being placed on an email blocklist (or blacklist). These are indicators that you might be increasing volume too quickly or that your list hygiene needs attention.
List segmentation is paramount during this phase. If you have a large list, sending to the most engaged segments (e.g., those who have opened or clicked in the last 30-60 days) minimizes risk. Avoid sending to dormant or unengaged subscribers during warm-up, as this can quickly damage your reputation on the new shared IPs. Consider segmenting your list into tiers based on engagement to manage volume increases effectively. This also applies when migrating while warming up.
The duration of the warm-up period depends on your sending volume and the historical engagement of your list. For a 25M monthly volume with a 900K contact list, expect the warm-up to take several weeks, potentially longer, especially if you encounter any deliverability issues. Patience is key. Rushing the process can lead to significant deliverability problems that are much harder to resolve later.
Don't rush the ramp-up
Even with a good domain and shared IPs, sudden volume spikes can negatively impact your sender reputation. A gradual increase allows mailbox providers to adjust to your new sending patterns and build trust with the new IP addresses. If you push too hard, you risk being flagged, which can severely hinder your deliverability for weeks or even months.
The length of your warm-up also depends on how consistently you've been sending from the old ESP. If you've had gaps in sending, or inconsistent volumes, the warm-up period might need to be more conservative. Mailbox providers value predictability and consistent, positive engagement.
Authentication and monitoring
Proper email authentication (SPF, DKIM, DMARC) is foundational for deliverability, and it's especially critical during an ESP migration. While your domain remains the same, the underlying configuration (e.g., DKIM keys, SPF records) will likely change to reflect your new ESP's infrastructure. Ensure these records are correctly configured in your DNS before you begin sending from the new ESP.
DMARC reports provide invaluable insights into your email authentication and deliverability performance during the migration. By analyzing these reports, you can identify any authentication failures (SPF, DKIM, or DMARC alignment) that might be occurring with your new setup. This visibility allows you to quickly troubleshoot and fix issues, preventing potential reputation damage. We offer DMARC monitoring to help you with this.
Beyond the initial warm-up, continuous monitoring of your email metrics is essential. Pay attention to inbox placement rates, open rates, click-through rates, unsubscribe rates, and spam complaint rates. Set up alerts for any unusual spikes in complaints or bounces, and regularly check for any unexpected blocklist (or blacklist) appearances. A proactive monitoring strategy ensures that you maintain a healthy sender reputation on your new shared IPs.
In conclusion, even when migrating to a new ESP with the same sending domain and shared IP addresses, a strategic warm-up process is absolutely necessary. It's about building trust with mailbox providers for your domain-IP combination in the new environment. By gradually increasing sending volume to your most engaged subscribers, meticulously monitoring your performance, and ensuring proper authentication, you can achieve a smooth transition and maintain strong email deliverability.
Views from the trenches
Best practices
Start by sending emails to your most engaged subscribers who consistently open and click, as this builds positive reputation signals.
Monitor key metrics closely: watch open rates, click rates, bounce rates, and spam complaint rates, adjusting your sending volume based on these indicators.
Ensure all email authentication records (SPF, DKIM, DMARC) are correctly configured for your new ESP before starting any sends.
Communicate proactively with your new ESP to understand their specific warm-up recommendations for shared IP pools.
Common pitfalls
Sending large volumes too quickly from the new ESP, which can trigger spam filters and result in blocklisting or lower inbox placement.
Neglecting to monitor your DMARC reports, missing critical insights into authentication failures or deliverability issues during the migration.
Sending to unengaged or old segments of your list during the warm-up period, which can lead to high bounce rates and spam complaints.
Assuming that existing domain reputation or shared IP warming means no additional warm-up is needed for your specific traffic.
Expert tips
Even with a strong domain and shared IPs, every new IP/domain combination needs an introduction to mailbox providers to establish trust.
The warm-up period for shared IPs can sometimes be shorter than for dedicated IPs, but it still requires a methodical approach.
Consider a phased migration, where you gradually shift segments of your audience to the new ESP, maintaining some volume on the old platform initially.
Pay close attention to changes in your DKIM signing domain, as Google and Yahoo increasingly rely on this for traffic identification.
Marketer view
It's important to remember that any new IP and domain combination will impact email delivery, necessitating a warm-up period to introduce the new setup to filters.
July 2024 - Email Geeks
Expert view
You don't need to start as slowly as was recommended in earlier years. Current general recommendations suggest beginning with around 3,000 to 5,000 emails for initial warm-up volumes.
Google and 
Yahoo to recognize and trust your email volume coming from the new ESP's infrastructure. It minimizes the risk of your emails landing in the spam folder or encountering other email deliverability issues.
