How does email reputation transfer during IP warming with a new ESP, and how to resolve deliverability issues?
Matthew Whittaker
Co-founder & CTO, Suped
Published 8 Aug 2025
Updated 19 Aug 2025
6 min read
Migrating to a new email service provider (ESP) is a significant undertaking. One of the most common questions that comes up during this process is how email reputation carries over, especially during the critical IP warming phase. Many assume that their established sender reputation will seamlessly transfer, but the reality is more nuanced.
While your domain reputation can certainly provide a foundational level of trust, the specific IP addresses you use play a massive role, and these usually change with a new ESP. This means a careful approach to IP warming is crucial to avoid deliverability issues, like your emails landing in the spam folder.
Understanding email reputation during ESP migration
When you switch ESPs, you're almost certainly going to be sending from new IP addresses. Internet Service Providers (ISPs) track reputation at multiple levels, including the IP address. A new IP address starts with no sending history, making it inherently suspicious until it builds a positive track record. This is why IP warming is so vital during a migration.
However, your domain reputation is generally more portable. If your domain has a strong history of good sending practices, low complaint rates, and high engagement, ISPs will factor this into their assessment. The key here is ensuring your domain authentication records, such as SPF, DKIM, and DMARC, are correctly configured for your new ESP.
Mailbox providers often assess a combination of factors, sometimes referred to as a 'tuple' of IP:selector:domain. Even with your domain reputation intact, the new IP and DKIM selector (if it changes) will need to establish their own positive standing. This is why you might see a temporary dip in deliverability despite a healthy domain.
The critical role of IP warming
IP warming (also known as sender warming or email warm-up) is the process of gradually increasing the volume of emails sent from a new IP address. The goal is to build a positive sending history with ISPs, proving you are a legitimate sender and not a spammer. Without proper warming, ISPs are more likely to flag your emails as suspicious, sending them straight to the spam folder or blocking them outright.
This process involves starting with a low volume of emails to your most engaged subscribers. Over time, you slowly increase the volume and broaden the audience. The key is to maintain high engagement rates and low complaint rates throughout this period. A positive engagement from recipients signals to ISPs that your mail is wanted.
Skipping or improperly executing IP warming can lead to severe deliverability issues. ISPs need to see consistent, positive sending behavior from a new IP before they fully trust it. As Microsoft Learn notes, mailbox providers view email from a new domain (or IP) as suspicious until a positive reputation is established. This phase is crucial for avoiding spam filters and ensuring your emails reach the inbox.
The danger of improper warming
I recently encountered a situation where an IP had been warmed, but then no emails were sent for three months. This effectively reset the IP's reputation, causing subsequent sends (like a welcome flow) to land in the spam folder, particularly with Gmail. A 0.2% spam rate, even if it seems low, is a concerning indicator that subscribers are actively marking your emails as unwanted, which can severely impact your sender score.
Common deliverability challenges and how to address them
Even with careful planning, you might encounter deliverability challenges when migrating. One common issue is emails landing in the spam folder, even for engaged subscribers. This often points to ISPs (like Gmail) needing more signals of positive engagement from your new sending infrastructure.
To mitigate this, start your warming process by sending to your most active and engaged subscribers first. These are the people most likely to open your emails, click links, and avoid marking them as spam. This positive interaction helps build the necessary reputation for your new IP. Gradually expand to less engaged segments as your reputation strengthens.
Seed testing is a valuable diagnostic tool during this period. Sending test emails to a variety of mailbox providers can help you identify specific inbox placement issues. If you notice emails going to spam, try to isolate the variable. Is it the IP? Is it the domain? Is it the content? As Spamhaus points out, your sender reputation cannot be simply transferred, it must be rebuilt.
IP reputation issues
If emails from your new IP address are hitting spam, but identical emails from a different, established IP inbox successfully, it indicates an IP reputation problem. This is a common challenge during warming.
Domain reputation issues
If emails fail even when sent from a reputable IP, but stop failing when the sending domain is removed or changed, your domain might have a poor reputation. In severe cases, just the presence of your domain in an email (even in a tracking pixel URL) can cause spam placement.
Best practices for a smooth transition
Beyond IP warming, a successful migration requires meticulous attention to technical setup. Ensure all your DNS records, particularly SPF, DKIM, and DMARC, are correctly configured for your new ESP before you start sending. Misconfigurations here can instantly damage your domain's sending ability.
Regularly monitor your sending metrics within your ESP and through external tools like Google Postmaster Tools. Pay close attention to complaint rates, bounce rates, and spam trap hits. If you see any spikes, pause your sending or adjust your warming schedule immediately. Continuous monitoring allows you to catch and address issues before they escalate into serious blacklists (or blocklists) or deliverability problems.
Finally, use the ESP migration as an opportunity to clean your email list. Removing unengaged subscribers, bounces, and potential spam traps before warming up your IP will significantly improve your chances of success. A clean list ensures you're only sending to recipients who genuinely want your emails, which is paramount for maintaining a good sender reputation.
Views from the trenches
Best practices
Start your IP warming by sending to your most engaged subscribers first. This builds initial trust with ISPs.
Ensure all DNS authentication records, including SPF, DKIM, and DMARC, are correctly configured for your new ESP.
Regularly monitor your email metrics, such as complaint rates and spam placement, using Postmaster Tools and ESP dashboards.
Common pitfalls
Assuming full sender reputation automatically transfers, leading to immediate high-volume sending from new IPs.
Neglecting to re-warm an IP address if it has been inactive for an extended period after initial warming.
Ignoring early warning signs like a slight increase in spam folder placement during the warming process.
Expert tips
Use seed testing to diagnose specific deliverability issues by isolating variables like IP, DKIM, or domain reputation.
Understand that mailbox providers often consider a combination of IP, DKIM selector, and domain reputation for delivery.
If your emails go to spam, consider whether the problem lies with the IP, the domain, or even the presence of your domain in tracking links.
Expert view
Expert from Email Geeks says that the machine learning algorithms of Google, Oath, and Microsoft operate differently, so there isn't one definitive answer regarding reputation transfer during an ESP migration.
2024-03-10 - Email Geeks
Expert view
Expert from Email Geeks states that if emails are DKIM-signed with the same parent domain at both the old and new ESP, some domain reputation is likely to carry over.
2024-03-12 - Email Geeks
Navigating your ESP migration for continued success
Navigating the complexities of email reputation during an ESP migration requires a strategic and patient approach. While your domain's established trust can help, the new IP addresses must build their own positive reputation through meticulous IP warming.
By focusing on engaged subscribers, rigorously monitoring your metrics, and maintaining proper authentication, you can minimize deliverability issues and ensure your emails continue to reach the inbox, safeguarding your sending reputation long-term.
