How to resolve email deliverability issues and IP blocks when migrating to a new ESP?
Matthew Whittaker
Co-founder & CTO, Suped
Published 16 Jul 2025
Updated 19 Aug 2025
8 min read
Migrating to a new email service provider (ESP) is a significant undertaking that can profoundly impact your email deliverability. While the goal is often to improve sending performance or access new features, the transition itself can introduce unexpected challenges, including temporary deliverability drops and even IP blocklists (or blacklists). Many senders find themselves facing issues like messages being blocked by mailbox providers (MPBs) shortly after the switch. It's a common concern, and navigating these waters requires a clear strategy.
The key to a smooth migration and resolving any issues that arise lies in understanding why deliverability hiccups occur and implementing best practices to mitigate risks. It is not uncommon for a new IP address, even one in a shared pool, to encounter initial scrutiny from ISPs, leading to messages being filtered or blocked. Knowing how to react, whether to hold back or push forward, is critical to maintaining your sender reputation.
Preparation is key: pre-migration steps
Before you even consider moving your email sending to a new ESP, foundational steps are required to ensure a healthy transition. A clean email list is paramount. Sending to disengaged or invalid addresses can instantly damage your new IP or domain reputation, making it harder to establish trust with mailbox providers. Cleaning your list proactively reduces bounce rates and spam complaints.
Another critical step is ensuring your domain's email authentication records are correctly configured. This includes SPF, DKIM, and DMARC. These protocols verify that your emails are legitimate and sent from authorized sources, which is essential for deliverability. Incorrectly configured records can lead to emails failing authentication and being rejected or sent to the spam folder. It's also vital to understand how SPF and DKIM authentication work together for optimal results.
Even with shared IP pools, where the ESP manages much of the IP reputation, your domain's reputation is still largely your responsibility. Preparing your domain by ensuring its history is clean and its authentication is solid will lay a strong foundation for your new sending infrastructure.
Pre-migration checklist
List hygiene: Clean your subscriber list rigorously to remove inactive or invalid addresses.
Authentication setup: Configure SPF, DKIM, and DMARC records for your sending domain.
Domain reputation: Ensure your domain has a positive sending history.
Content review: Audit your email content for potential spam triggers.
Strategic IP warming and reputation building
IP warming (or ramp-up) is the process of gradually increasing the volume of email sent from a new IP address. This helps mailbox providers recognize your sending patterns and build a positive reputation for your new IP. While crucial for dedicated IPs, even with shared IPs, your sending volume contributes to the pool's reputation, and aggressive sending can trigger filters or blocklists. A comprehensive guide to IP and domain warming emphasizes this gradual approach.
When migrating to a new ESP, especially if it involves a new IP address (dedicated or shared), a structured warming plan is essential. Start with your most engaged subscribers, as positive engagement signals (opens, clicks) are vital for building a good sender reputation. Gradually increase your sending volume over several weeks, carefully monitoring your deliverability metrics. It is important to know what is the best IP warm-up strategy to ensure a smooth transition.
A common mistake is rushing the warming process. If you encounter blocks, like the Microsoft S3140 block (a specific blocklist from Outlook.com, often related to reputation issues on a shared IP), do not immediately try to resend the blocked messages from another system. This can be interpreted as snowshoe spamming, where a sender uses multiple IPs or domains to bypass filters, and can further harm your reputation. Instead, pause your sending, analyze the block message, and adjust your strategy.
In cases of shared IP blocks, the ESP is often responsible for resolving the issue, as their network's overall reputation affects all users. While they might say it will improve over time, a proactive ESP will work to get their IP blocklisted (or blacklisted) issues resolved. It's crucial to understand whether IP warming is necessary for shared IPs.
IP warm-up volume examples
Sample IP Warm-up Schedule
Day 1-3: 5,000 emails
Day 4-7: 10,000 emails
Day 8-14: 25,000 emails
Day 15-21: 50,000 emails
Day 22+: Gradual increases towards full volume
This is a general guideline. Actual volumes should be adjusted based on your list size, engagement, and real-time deliverability feedback.
Troubleshooting and mitigating IP blocklists
When confronted with IP blocks or significant deliverability drops after migrating, a systematic approach to troubleshooting is essential. The first step is to analyze the bounce messages. They often contain specific error codes or URLs (like Microsoft's troubleshooting page) that provide clues about the underlying problem, such as being listed on a specific blocklist or having a low sender score. Knowing what happens when your IP gets blocklisted can guide your response.
If the issue is with a shared IP, open a ticket with your new ESP's support team. They are generally responsible for managing the reputation of their shared IPs and should actively work to delist any IPs on a blocklist. Your role is to ensure your sending practices align with their policies and industry best practices. They will need to identify if other senders in the shared pool are causing issues.
For your domain, check if your DKIM signing is correct and if your return path is properly whitelisted (often called custom return path or bounce domain). These configurations can significantly influence how mailbox providers assess your sender trustworthiness, especially when using a shared ESP IP pool. I recommend using a tool to test your email deliverability.
Recommended actions
Analyze bounce messages for specific error codes or blocklist names. Pause sending to affected mailbox providers temporarily if blocks occur. Contact your ESP to investigate shared IP blocklist issues. Verify all DNS authentication records (SPF, DKIM, DMARC) are correctly set up and aligned. Reduce sending volume and focus on re-engaging your most active subscribers during recovery. Improve email content quality and personalization to boost engagement. Monitor your IP and domain blocklist status regularly.
Actions to avoid
Blindly resending blocked emails, especially from a different ESP or domain, as this can trigger snowshoe detection and further reputation damage. Ignoring bounce messages or hoping issues will resolve themselves without intervention. Rushing the IP warming process or immediately sending high volumes of emails from a new IP. Sending to unengaged subscribers or purchased lists, which can lead to spam traps and complaints. Neglecting email authentication, as it's a fundamental trust signal for ISPs. Overlooking the importance of domain reputation, focusing solely on IP reputation.
Ongoing monitoring and reputation management
After addressing immediate blocks and adjusting your sending strategy, continuous monitoring is crucial for long-term deliverability success. Keep a close eye on your bounce rates, complaint rates, and open rates. High bounce rates might indicate a need for more aggressive list cleaning, while high complaint rates signal content or list engagement issues. Reviewing your DMARC reports from Google and Yahoo offers invaluable insights into authentication failures and deliverability.
Regularly check common IP and domain blocklists (blacklists) to ensure your sending infrastructure remains clear. Many ESPs provide tools or dashboards for this, but independent monitoring can offer a broader view. Remember, being listed on a blocklist can severely impact your ability to reach the inbox, so proactive checking is key. Understanding the different types of email blocklists helps in targeted troubleshooting.
Maintaining consistent sending volume and good engagement is essential. Avoid large, erratic sending spikes, which can look suspicious to mailbox providers. Keep your content relevant and engaging to encourage opens and clicks, which positively signal to ISPs that your emails are valued by recipients. This helps build a strong sender reputation over time, reducing the likelihood of future blocks. To fully understand the challenges of switching ESPs, consider a holistic approach.
Key deliverability metrics to monitor
Bounce rate: Keep below 2-3%. High rates indicate list quality issues.
Complaint rate: Aim for less than 0.1%. Signals recipients find your content unwanted.
Open and click rates: Higher rates show engagement and positive sender reputation.
Blocklist status: Regularly check IP and domain for listings on common blacklists.
DMARC reports: Analyze these reports for authentication failures and delivery issues.
Views from the trenches
Best practices
Always clean your email list thoroughly before migrating to a new ESP to avoid carrying over problematic addresses.
Implement a structured IP warming plan, starting with your most engaged subscribers and gradually increasing volume.
Ensure all email authentication protocols (SPF, DKIM, DMARC) are correctly configured for your sending domain.
Proactively monitor your deliverability metrics, including bounce rates, complaint rates, and IP/domain blocklist status.
Maintain consistent sending volume and send engaging content to foster positive sender reputation signals.
Communicate openly with your new ESP's support team about any deliverability issues, especially IP blocks.
Common pitfalls
Resending previously blocked emails from a different ESP, which can trigger snowshoe spam detection and worsen reputation.
Ignoring bounce messages or expecting deliverability issues to resolve on their own without intervention.
Rushing the IP warming process, which can lead to immediate filtering or IP blocklistings (blacklisting).
Failing to address the root causes of deliverability problems, such as poor list quality or low engagement.
Underestimating the impact of domain reputation compared to IP reputation during the migration.
Not having a plan for managing deliverability after the initial migration phase.
Expert tips
Use a dedicated subdomain for marketing emails on your new ESP to isolate its reputation from your primary domain.
Leverage DMARC aggregate reports to gain detailed insights into your email authentication and delivery performance.
Segment your list based on engagement and target highly engaged subscribers during the initial warm-up phases.
Be prepared to pause sending or adjust volumes if you see a sudden drop in deliverability or an increase in blocks.
Understand that even with shared IPs, your sending behavior directly contributes to the pool's overall reputation.
Regularly review your email content for spam triggers and ensure it provides clear value to recipients.
Expert view
Expert from Email Geeks says that blindly resending messages is never a good idea. Senders need to find out why the messages were blocked first before taking any action.
2019-11-07 - Email Geeks
Expert view
Expert from Email Geeks says that resending previously rejected messages from a different ESP will likely make you appear as a spammer to mailbox providers.
2019-11-07 - Email Geeks
Charting a successful migration course
Migrating to a new ESP doesn't have to be a daunting task, but it requires diligent planning and execution. By focusing on meticulous preparation, strategic IP warming, careful troubleshooting of any deliverability issues (including IP blocklists), and continuous monitoring, you can minimize disruption and ensure your email program continues to thrive. Remember, building and maintaining a positive sender reputation is an ongoing effort that pays dividends in inbox placement.
The initial period on a new platform is crucial for establishing trust with mailbox providers. While temporary setbacks like blocks can occur, a calm and analytical approach, combined with adherence to best practices, will guide you through successfully. Prioritize your list quality, proper authentication, and consistent monitoring to make your ESP migration a success story.
Microsoft S3140 block (a specific blocklist from Outlook.com, often related to reputation issues on a shared IP), do not immediately try to resend the blocked messages from another system. This can be interpreted as snowshoe spamming, where a sender uses multiple IPs or domains to bypass filters, and can further harm your reputation. Instead, pause your sending, analyze the block message, and adjust your strategy.
