Should I mail unengaged users during an ESP migration?
Michael Ko
Co-founder & CEO, Suped
Published 21 Apr 2025
Updated 16 Aug 2025
7 min read
Migrating your email service provider (ESP) is a significant undertaking, often seen as a fresh start for your email program. It's natural to eye those unengaged segments of your list, wondering if this transition is an opportune moment to try and re-ignite their interest. You might think, "If deliverability issues arise on the old ESP, they won't follow us to the new one, right?" This line of thinking, however, carries considerable risks for your email deliverability and sender reputation.
The impulse to re-engage inactive subscribers is understandable. Every email address represents a potential customer or loyal reader, and nobody wants to leave potential revenue on the table. However, it's crucial to approach this with caution, especially during a sensitive period like an ESP migration.
Your sender reputation is closely tied to your domain, not just your specific IP addresses or the ESP you use. While you'll be moving to new IPs and subdomains, your primary sending domain remains the same. Mailbox providers (like Google and Yahoo) employ sophisticated algorithms that can connect sending behavior across different IPs or subdomains if they originate from the same primary domain. This means any negative signals generated from aggressive sending to unengaged users on your old ESP could still impact your sender reputation on the new platform.
The risks of mailing unengaged contacts
Sending to unengaged users, especially those who haven't interacted with your emails in a long time, carries inherent risks. These risks are amplified during an ESP migration when you are trying to establish a new sending reputation on new infrastructure. Even if you are moving from old dedicated IPs and subdomains to new ones, your primary sending domain's reputation is still very much in play.
One of the biggest concerns is hitting spam traps. These are email addresses specifically set up by mailbox providers and blocklist (or blacklist) operators to identify senders with poor list hygiene. Sending to a spam trap will immediately flag your sending practices as suspicious or abusive, leading to severe deliverability problems, including being added to an email blocklist. For more information on these, read our guide on spam traps.
Engagement metrics are paramount to your sender reputation. Low open rates, high bounce rates, and increased complaint rates (when users mark your emails as spam) signal to mailbox providers that your emails are not desired. This can quickly degrade your sender reputation, regardless of which ESP you are using. If you email inactive users, your deliverability to engaged users suffers.
Think of it this way: mailbox providers want to deliver emails that recipients want to receive. If your emails are consistently ignored or marked as spam, it tells them your content isn't relevant. This can lead to your emails being diverted to the spam folder or blocked entirely, even for your most active subscribers. Learn more about the risk of sending to inactive users.
Why it's especially risky during an ESP migration
Risky approach: mailing unengaged users
Reputation Damage: Leads to increased spam complaints and lower engagement rates on your old ESP, which can still negatively impact your domain reputation overall.
Spam Trap Risk: Higher chance of hitting hidden spam trap addresses, potentially leading to blacklisting of your domain.
Migration Complications: Disrupts the crucial IP warming process on the new ESP, making it harder to build a positive reputation.
During an ESP migration, you are essentially building a new sender reputation from scratch on your new IP addresses and subdomains. This process, known as IP warming or domain warming, requires sending to your most engaged subscribers first and gradually increasing your sending volume. Introducing unengaged users into this delicate process can severely jeopardize your efforts.
If you send to unengaged contacts from your old ESP, even if you are transitioning away from it, you risk creating a negative feedback loop for your domain. Mailbox providers see patterns. A sudden spike in complaints or bounces linked to your domain, even from an old sending infrastructure, can still flag your domain as risky. This history can, in turn, make it harder for your new IPs and subdomains to establish a clean reputation.
Focusing on establishing a strong, positive sending reputation on your new ESP is paramount. This means nurturing your most active subscribers with relevant content. Any attempt to "clean out" or "re-engage" unengaged users during this critical phase can derail your entire migration and lead to long-term deliverability problems.
Strategic management of unengaged users
The risks of mailing unengaged users during ESP migration
Spam traps: Increased likelihood of hitting spam traps, leading to blocklistings (blacklistings) for your domain.
Warming process: Disrupts the delicate IP/domain warming needed for your new infrastructure.
Instead of risking your established reputation and complicating your migration, implement a structured approach to managing unengaged users. First, ensure your email list is clean before migrating. This means removing hard bounces, unknown users, and clearly unengaged subscribers based on your defined criteria. Consider reading Steve Henderson's perspective on inactive contacts.
Once your migration is complete and your new IPs/domains are properly warmed up with engaged traffic, then you can devise a separate re-engagement strategy for a segmented portion of your unengaged list. This might involve a targeted campaign with compelling offers or content, sent in a carefully managed, incremental fashion. Monitor your metrics closely: open rates, click-through rates, and especially complaint rates. If the engagement is still poor, it's often best to remove these subscribers to protect your overall sender reputation. You can also define unengaged subscribers for removal.
Remember, the goal is not just to send emails, but to send emails that reach the inbox and get opened. Sending to unengaged contacts typically works against this goal. Maintaining a pristine sender reputation should always be your top priority.
Consider the differences between an immediate, risky approach and a planned, strategic one:
Immediate sending of unengaged users
Deliverability Impact: High risk of spam folder placement or blocks, compromising new IP/domain warming.
Reputation Risk: Damages overall domain reputation, potentially leading to long-term issues.
Cost: Wasted sends and potential for higher ESP costs due to larger list sizes with low ROI.
Strategic re-engagement post-migration
Deliverability Impact: New IPs/domains warm up effectively, ensuring high inbox placement for engaged users.
Reputation Protection: Domain reputation remains strong, as you only send to engaged users initially.
Cost: Optimized sending to a valuable list, leading to better ROI.
Views from the trenches
Best practices
Start your ESP migration by sending only to your most active and engaged subscribers to build a solid reputation.
Segment your unengaged users and plan a separate re-engagement campaign for *after* your migration and warming are complete.
Define clear criteria for unengaged users (e.g., no opens/clicks in 90-180 days) and remove them if re-engagement fails.
Regularly monitor your email engagement metrics and sender reputation using tools like Google Postmaster Tools.
Prioritize list hygiene. Sending clean, targeted emails is always better than mass sending to a stale list.
Common pitfalls
Attempting to re-engage a large segment of unengaged users immediately after switching ESPs.
Believing that issues on your old ESP's IPs will not impact your primary sending domain reputation on the new ESP.
Failing to clean your list of hard bounces, unknown users, and long-term inactives before migrating.
Ignoring high complaint rates or low engagement during migration, thinking it's a temporary issue.
Not having a clear sunset policy for subscribers who remain unengaged after re-engagement efforts.
Expert tips
Even if your current sender reputation is stellar, aggressive sending to unengaged users during migration can quickly erode it.
Mailbox providers use sophisticated algorithms that can link sending behavior across different IPs/subdomains under the same primary domain.
Re-engaging inactive users should be a carefully considered, ongoing strategy, not a one-off bulk send, especially not during a migration.
Focus on quality over quantity. A smaller, highly engaged list is far more valuable than a large, stagnant one.
Always track your results. If a re-engagement effort isn't yielding positive results, stop and prune your list.
Expert view
Expert from Email Geeks says to assume that mailbox providers know when senders are engaging in questionable sending practices.
2022-03-24 - Email Geeks
Marketer view
Marketer from Email Geeks says that the benefits of mailing to inactive contacts are largely fictional.
2022-03-24 - Email Geeks
Key takeaways
The short answer is no: you should not mail unengaged users during an ESP migration. While the idea of a fresh start on new IPs and subdomains is appealing, your domain's reputation is persistent. Any negative sending behavior, even from your old ESP, can still follow your domain and compromise your new sending infrastructure. The primary goal during an ESP migration should be to establish a solid, positive sending reputation with your most engaged subscribers on your new platform.
By prioritizing list hygiene, segmenting your audience, and strategically planning any re-engagement efforts for after your migration and warming process are complete, you'll maximize your chances of a successful transition and long-term email deliverability success. Avoid the temptation to take shortcuts that could lead to your emails ending up in the spam folder or on a blocklist (or blacklist).
Google and 
Yahoo) employ sophisticated algorithms that can connect sending behavior across different IPs or subdomains if they originate from the same primary domain. This means any negative signals generated from aggressive sending to unengaged users on your old ESP could still impact your sender reputation on the new platform.
