What are the best practices for migrating to a new ESP while retaining sender reputation?
Matthew Whittaker
Co-founder & CTO, Suped
Published 22 Jul 2025
Updated 19 Aug 2025
7 min read
Migrating to a new Email Service Provider (ESP) is a significant undertaking for any business. It involves more than just transferring your contact lists and email templates. A critical aspect that often causes anxiety is the potential impact on your sender reputation, which is painstakingly built over time with mailbox providers like Gmail and Outlook. Maintaining your good standing is crucial to ensure your emails continue to land in inboxes, not spam folders.
When you switch ESPs, you often move to new IP addresses, which can effectively reset your sender reputation. The goal is to carefully manage this transition to minimize disruption and quickly establish a positive reputation with your new provider. By following a structured approach, you can navigate this complex process effectively and preserve your hard-earned deliverability.
Pre-migration preparations
Before you even begin to move data, thorough preparation is essential. This foundational step dictates the smoothness of your entire migration and your ability to retain sender reputation.
Clean your list first
The first, and arguably most important, step is to clean your email list. Removing unengaged subscribers, hard bounces, and known spam traps from your list before migrating will significantly improve your initial sending reputation on the new platform. A clean list ensures that your first sends from the new ESP go to highly engaged recipients, signaling positive intent to mailbox providers.
This practice is vital because sending to bad addresses or disengaged users can quickly damage your new IP's reputation and lead to blocklisting (or blacklisting). Take the time to identify unreachable contacts and review engagement rates from your current ESP to ensure you are only migrating a healthy, active subscriber base. This proactive approach sets a strong foundation for your future deliverability.
Benchmark current performance
Before switching, capture detailed benchmarks of your current email program's performance. This includes key engagement metrics such as open rates, click-through rates, unsubscribe rates, and spam complaint rates, broken down by Internet Service Provider (ISP). This data will serve as a crucial baseline to measure the success of your migration and identify any deviations quickly. Without these benchmarks, it is challenging to assess the impact of the migration accurately. You can use tools like Google Postmaster Tools or your existing ESP's analytics to gather this information.
Technical configuration and authentication
Proper technical configuration is non-negotiable for maintaining sender reputation during an ESP migration. Incorrectly set up authentication can lead to immediate deliverability issues.
Domain authentication setup
One of the most critical steps is to set up your domain authentication records: SPF (Sender Policy Framework), DKIM (DomainKeys Identified Mail), and DMARC (Domain-based Message Authentication, Reporting, and Conformance) with your new ESP. These protocols verify that your emails are legitimately sent from your domain, preventing spoofing and improving trust with mailbox providers.It is essential to update your Domain Name System (DNS) records to reflect the new ESP's sending infrastructure. Ensure these are correctly configured and propagated before you begin sending significant volumes of email.
You should also strongly consider migrating your existing sending subdomain to the new ESP. Keeping the same sending domain (or subdomain) can help preserve your accumulated domain reputation, as many mailbox providers prioritize domain reputation over IP reputation, especially with Google. However, be aware that you might not be able to use the same subdomain on both systems in parallel, depending on your new ESP's delegation process.
Example SPF record for your new ESPTXT
v=spf1 include:_spf.newesp.com ~all
For DMARC, if you don't already have one, use a DMARC record generator and start with a p=none policy. This allows you to monitor your email streams and ensure proper authentication without risking deliverability issues. Once confident in your authentication, you can gradually move to p=quarantine or p=reject to enforce stricter protection. Review your DMARC reports regularly for any authentication failures.
IP and domain warming
One of the most critical phases of an ESP migration, especially if you are using new dedicated IP addresses, is the IP and domain warming process. This involves gradually increasing your sending volume to allow mailbox providers to establish a positive reputation for your new IPs.
IP warming strategies
During IP warming, send initially to your most engaged subscribers. This ensures high open and click rates, which sends positive signals to ISPs. Gradually increase your sending volume and expand to less engaged segments over several weeks. Your ESP should provide a detailed IP warming schedule tailored to your list size and sending frequency. Avoid rushing this process, as it can severely impact your deliverability and lead to immediate blacklisting (or blocklisting).
Dedicated IP addresses
When you use dedicated IP addresses, you are solely responsible for building and maintaining the reputation of those IPs. This requires a careful and controlled IP warming process, where you gradually increase your sending volume over time. If not done correctly, you risk immediate deliverability issues, as your new IPs have no prior sending history and reputation.
The benefit is full control over your reputation, which is ideal for high-volume senders with consistent email practices. However, it demands patience and strict adherence to warming protocols.
Shared IP addresses
With shared IP addresses, you leverage the existing reputation of the ESP's sending infrastructure. While IP warming is still advisable to acclimatize your domain, the initial reputation impact is often less severe because you benefit from the collective sending history of other users. This can be beneficial for small send volumes or those with less consistent sending schedules.
The drawback is that your deliverability can be affected by the sending practices of other users sharing the same IPs. If another sender on a shared IP block gets blacklisted, your emails may also be impacted, even if your own sending practices are pristine.
Regardless of whether you use dedicated or shared IPs, it's often best to run both systems in parallel for as long as possible. This allows you to gradually shift your sending volume to the new ESP without a sudden drop in email deliverability. Start with transactional emails, which typically have higher engagement rates, before moving marketing campaigns.
Ongoing monitoring and maintenance
Even after successfully migrating and warming your IPs, continuous monitoring and adherence to best practices are crucial for long-term sender reputation health.
Continuous monitoring and feedback loops
Monitor your deliverability metrics closely on the new ESP. Pay attention to open rates, click-through rates, bounce rates, and spam complaint rates. Set up feedback loops (FBLs) with major ISPs like Microsoft and Yahoo to receive notifications when subscribers mark your emails as spam. This allows you to promptly remove those users from your list, preventing further damage to your reputation. Additionally, regularly check blocklist monitoring services to ensure your new IPs or domain haven't been listed.
Content and engagement
Beyond technical setup, the quality of your content and user engagement remain paramount. Continue to send relevant, valuable content to your subscribers. Encourage interaction through clear calls to action and personalized messaging. A/B test your emails on the new platform to assess deliverability and engagement. Consistent sending of engaging content to an active list will help solidify your sender reputation and ensure long-term deliverability success.
Views from the trenches
Best practices
Start by meticulously cleaning your email list to ensure you only migrate engaged subscribers, which helps build a positive sender reputation from day one.
Establish domain authentication records (SPF, DKIM, DMARC) with your new ESP well in advance to ensure proper email verification.
Implement a gradual IP and domain warming plan, starting with your most engaged segments and slowly increasing volume over time.
Run both your old and new ESPs in parallel during the migration phase to avoid abrupt changes in sending volume or reputation.
Benchmark your current deliverability and engagement metrics to accurately measure the success of your migration against a baseline.
Common pitfalls
Migrating your entire email list at once without cleaning it, which can lead to high bounce rates and spam complaints on new IPs.
Rushing the IP warming process, causing mailbox providers to flag your new IPs as suspicious due to sudden high volume.
Failing to properly configure SPF, DKIM, and DMARC records for your new ESP, leading to authentication failures and spam placement.
Neglecting to monitor deliverability metrics and feedback loops during and after migration, missing early signs of issues.
Underestimating the time and effort required for a successful migration and reputation rebuild, especially for dedicated IPs.
Expert tips
Ensure your chosen ESP provides strong onboarding support and guidance on IP warming and reputation management.
Segment your audience by engagement level and prioritize sending to your most active subscribers during the initial warming phase.
Leverage subdomains for different email types (e.g., transactional vs. marketing) to help isolate and manage email reputation.
Pay close attention to provider-specific requirements, especially for major mailbox providers like Google and Microsoft.
Continuously monitor for blocklist listings and address any issues promptly to prevent long-term damage to your sender reputation.
Marketer view
Marketer from Email Geeks says the most common pitfall when migrating to a new ESP is moving to new IP addresses, which requires significant time to regain the sender reputation already established.
2019-10-09 - Email Geeks
Marketer view
Marketer from Email Geeks says that with Google, it might be possible to get away with a smoother transition if new IPs are phased in slowly and the same sending domain is maintained, as Google primarily focuses on domain reputation.
2019-10-09 - Email Geeks
Ensuring a seamless ESP migration
Migrating to a new ESP doesn't have to be a daunting task that jeopardizes your sender reputation. With careful planning, a focus on list hygiene, proper technical configuration, and a strategic IP warming process, you can ensure a smooth transition. Remember that maintaining a positive sender reputation is an ongoing effort that extends beyond the migration itself.
By following these best practices, you can safeguard your email deliverability, ensuring your messages continue to reach your audience effectively from your new platform. Prioritize engagement, monitor your metrics, and adapt your sending strategy as needed to keep your emails out of the spam folder and in the inbox.
or your existing ESP's analytics to gather this information.
Microsoft and 
Yahoo to receive notifications when subscribers mark your emails as spam. This allows you to promptly remove those users from your list, preventing further damage to your reputation. Additionally, regularly check blocklist monitoring services to ensure your new IPs or domain haven't been listed.
