Should you continue sending emails from an old ESP during migration and warmup?
Michael Ko
Co-founder & CEO, Suped
Published 27 Jul 2025
Updated 18 Aug 2025
6 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 list. A critical question that often arises during this process is whether you should continue sending emails from your old ESP while you are warming up your new platform. This decision has major implications for your email deliverability, sender reputation, and overall business continuity.
The short answer is often yes, but with careful strategy and execution. Transitioning abruptly can severely impact your sender reputation, leading to emails landing in spam folders or being blocked entirely. Understanding how to manage this overlap effectively is key to a successful migration and maintaining strong relationships with your subscribers.
Why dual sending is often essential
Continuing to send from your old ESP while warming up the new one is not just advisable; it's often a necessity, especially for organizations with high sending volumes. Imagine the revenue impact if a large e-commerce business suddenly stopped all email communication for weeks or months during a migration. Maintaining active campaigns on the old platform ensures that your business operations continue uninterrupted, preserving vital revenue streams.
The primary reason for this parallel approach is to protect your sender reputation. Internet Service Providers (ISPs) like Gmail and Outlook monitor sending patterns closely. A sudden halt in sending from a known IP address, followed by a sudden burst from a new, unfamiliar one, can trigger spam filters. This is precisely why IP warming is critical, even when migrating to shared IPs.
Maintaining a consistent sending volume from your established platform helps signal to ISPs that you are a legitimate sender. This continuity prevents a sudden drop in your domain's reputation score and reduces the likelihood of being placed on a blacklist (or blocklist) during the transition phase. This also allows you to carefully manage the transition of your most engaged subscribers first.
By keeping both systems active, you provide a fallback. If you encounter unexpected deliverability issues with the new ESP during warmup, you can temporarily increase volume through the old platform to ensure your messages still reach your audience. This redundancy is a crucial safety net for any email program.
Strategic implementation of dual sending
A phased approach is the most effective way to migrate. Start by gradually shifting your most engaged contacts to the new ESP. These are the subscribers who consistently open and click your emails, providing positive engagement signals to ISPs on your new IP address. As your sender reputation builds on the new platform, you can begin to introduce less engaged segments.
Synchronizing your subscriber lists and, most importantly, your opt-outs across both platforms is paramount. Failing to do so can result in sending emails to unsubscribed users, which is a major red flag for ISPs and can lead to spam complaints and blocklisting (or blacklisting). This is also a good opportunity to clean your email list before migrating.
The duration of the overlap depends on your sending volume and the size of your lists. For many businesses, a 2-3 month overlap is sufficient. However, for very large senders, this period could extend to six months or even longer to ensure a thorough IP and domain warm-up without risking deliverability.
Planning your overlap strategy
Gradual shift: Begin by sending only to your most engaged contacts on the new ESP.
Data sync: Implement robust mechanisms to sync opt-outs and bounces.
Monitoring: Constantly monitor deliverability metrics for both platforms.
Contingency: Be prepared to adjust sending volumes if issues arise.
A well-executed overlapping period allows ISPs to recognize your sending domain and new IP addresses as trustworthy. This steady ramp-up helps build a positive sending history with ISPs, significantly reducing the risk of your emails being flagged as spam.
Key considerations for a smooth migration
One often overlooked aspect during ESP migration is the longevity of your old ESP's links. Many ESPs use their own tracking domains for links within your emails. When you cease your contract with them, these links might stop working, rendering old emails with broken calls to action or tracking. Before stopping sending from your old ESP, verify how long their tracking links will remain active. If they expire immediately, you should stop sending from the old platform well before the contract ends to prevent broken links in previously sent campaigns.
Beyond active sending, consider how to handle any triggered emails or automations. Do you stop them mid-completion on the old ESP or allow them to run their course? This decision depends on the complexity of your automation setup and the capabilities of your new ESP. Some businesses prefer to complete existing customer journeys on the old platform, while others transition all automations to the new system simultaneously to simplify management.
Continuous monitoring of your email deliverability during the migration is non-negotiable. Utilize tools to track your inbox placement, open rates, click-through rates, and bounce rates for both platforms. Keep a close eye on any spam complaint rates and blocklist (blacklist) appearances. This data will guide your sending volume adjustments on the new ESP.
Risks of stopping old ESP too soon
Damaged sender reputation: ISPs see sudden volume shifts as suspicious.
Lost revenue: Critical email communications halted during warmup.
Broken links: Old tracking links may become inactive unexpectedly.
Increased spam complaints: Lack of consistent engagement data for new IPs.
Business continuity: No interruption in essential email campaigns or revenue.
Data collection: Gather performance data from the new ESP before full cutover.
Reduced stress: A backup system provides peace of mind during a complex transition.
By carefully managing these aspects, you can ensure a smoother ESP migration that protects your sender reputation and keeps your email program running efficiently. This dual-sending strategy is an investment in your long-term email deliverability success.
Views from the trenches
Best practices
Maintain consistent sending volumes from your old ESP during the initial warm-up period of the new platform.
Prioritize migrating your most engaged segments to the new ESP first to build positive reputation swiftly.
Implement robust and continuous synchronization of opt-outs between both ESPs to prevent unwanted sends and complaints.
Common pitfalls
Abruptly stopping email sends from the old ESP, which can significantly damage sender reputation and deliverability.
Failing to sync unsubscribes and bounces between platforms, leading to spam traps and blocklist hits.
Not accounting for the expiration of old ESP tracking links, resulting in broken links in past emails.
Expert tips
Always verify the longevity of tracking links from your old ESP before full decommissioning to ensure continuity.
Consider a staged migration for different email types, such as marketing emails first, then transactional emails.
Use comprehensive deliverability monitoring tools to track performance on both platforms during the transition.
Expert view
Expert from Email Geeks says that operating both platforms in tandem is standard practice, especially for large clients with high sending volumes, to ensure revenue continuity during migration. It is advisable to gradually shift the most engaged traffic to the new platform first.
June 22, 2024 - Email Geeks
Expert view
Expert from Email Geeks says that maintaining the old provider for a few months offers a crucial backup during migration and provides valuable data for warming up the new platform and measuring success. Synchronizing opt-outs between the two platforms is essential for a smooth warm-up.
June 22, 2024 - Email Geeks
Ensuring a seamless transition
The decision to continue sending emails from your old ESP during a migration and warm-up period is almost always beneficial, if not essential. It serves as a bridge, maintaining your sender reputation, ensuring business continuity, and providing a safety net against unforeseen deliverability challenges.
By following a carefully planned, gradual transition, synchronizing critical data, and continuously monitoring your performance, you can navigate your ESP migration smoothly. This strategic approach minimizes risks and sets your new email program up for long-term success.
