How to diagnose email deliverability drops after migrating to a new ESP?
Matthew Whittaker
Co-founder & CTO, Suped
Published 23 May 2025
Updated 19 Aug 2025
5 min read
Migrating to a new Email Service Provider (ESP) can sometimes lead to unexpected drops in email deliverability. This can be a frustrating experience, as the goal of switching ESPs is often to improve performance, not hinder it. It's a common scenario where emails that once landed reliably in inboxes suddenly start hitting spam folders or getting blocked. Diagnosing these issues requires a systematic approach, examining various factors from technical configurations to sending practices and recipient engagement.
Understanding the immediate signs
The first step in diagnosing a deliverability drop is to meticulously review your new ESP's reporting. Pay close attention to metrics that might indicate a problem. A noticeable decline in the delivered percentage or an increase in bounces, deferrals, or blocks are clear red flags. Some ESPs might define delivered differently, so understanding their specific metric definitions is crucial. For instance, if you see a drop in deliverability to specific providers, like Gmail addresses, it suggests a targeted issue with that particular Mailbox Provider.
Without granular reporting, diagnosing the exact cause can be challenging. You should ideally be able to see detailed breakdowns by Internet Service Provider (ISP), bounce codes, and reasons for blocks or deferrals. This information points you towards specific problems, whether it's a content filter, a specific domain's blocking policy, or an issue with your sending reputation. A comprehensive understanding of your ESP's reporting interface will save you time in the long run.
Key metrics to prioritize
Delivered rate: The percentage of emails that successfully reach the recipient's server.
Bounces: Monitor hard bounces (permanent failures) and soft bounces (temporary failures).
Blocks: Check for instances where mailbox providers explicitly reject your emails, often due to reputation or content.
Complaints: A high complaint rate is a major red flag for deliverability and sender reputation.
Unsubscribes: While good for compliance, a sudden spike might indicate content or frequency issues.
Engagement metrics: Open and click rates can provide insights into how recipients perceive your emails.
Checking your technical setup
Technical misconfigurations are a frequent culprit for deliverability drops after an ESP migration. It's essential to verify that your Domain Name System (DNS) records, particularly those related to email authentication, are correctly set up on your new ESP. Incorrect or missing SPF, DKIM, and DMARC records can severely impact your sender reputation and lead to emails being sent to spam.
Even a minor typo in a TXT record can break authentication. Ensure that your new ESP's instructions for setting up these records were followed precisely. Use an online tool to check your email authentication for proper configuration and alignment. You can learn more about these authentication protocols and how they work in a simple guide to DMARC, SPF, and DKIM.
Previous configuration for DNS
SPF record: Included old ESP's sending servers.
DKIM key: Authenticated emails sent through the old ESP.
DMARC policy: Monitored authentication results for domain.
Custom tracking domains: Set up for link and open tracking.
New configuration issues to verify
Missing DNS records: Crucial SPF, DKIM, or DMARC entries are absent.
Incorrect entries: Typos or incorrect values in TXT records.
DMARC alignment failure: SPF or DKIM domains not aligning with your From domain.
Mismatched CNAMEs: Issues with custom tracking domain setup.
A crucial, yet often overlooked, aspect of ESP migration is the impact on your sender reputation. When you move to a new ESP, especially with a new dedicated IP address, you start with a relatively unknown reputation. This necessitates an IP warming process, where you gradually increase your sending volume to build trust with Mailbox Providers. If this isn't done correctly, you'll likely see emails land in the spam folder.
Your list quality also plays a significant role. Before migrating, it's vital to clean your email list by removing old, inactive, or invalid addresses. These can include addresses that have previously hard bounced or resulted in spam complaints. Sending to such addresses can trigger spam traps and quickly land your new IP or domain on a blocklist (or blacklist), as described in our guide on what happens when your domain is on an email blacklist.
Avoid these list migration mistakes
Not cleaning your list: Migrating old, unengaged, or invalid addresses to your new ESP.
Ignoring bounces/unsubscribes: Not suppressing these from your old list before importing.
Sending too fast: Skipping or rushing the IP warming process.
Gradually building trust with mailbox providers over time.
List hygiene
Removing invalid or unengaged addresses improves reputation.
Engagement
Positive interactions like opens and clicks boost inbox placement.
Complaint rate
High rates signal spam and lead to blocklists and direct blocking.
Spam trap hits
Indicate poor list acquisition practices, leading to severe penalties.
Reviewing content and sending practices
Changes in your email content or sending patterns can also negatively affect deliverability. If you've made significant alterations to your email templates, subject lines, or call-to-actions post-migration, these could be triggering spam filters. Mailbox Providers (ISPs) analyze content for spammy keywords, suspicious links, or unusual formatting. Consistency with your previous successful campaigns is generally a safe bet during the transition period.
Furthermore, review your sending frequency and audience segmentation. Are you suddenly sending more emails or to a much broader, less engaged audience? This can lead to increased complaints and lower engagement rates, both of which damage your sender reputation. Ensure your unsubscribe process is clear and easy, as frustrated recipients are more likely to mark your email as spam, which can explain why emails go to spam.
Optimizing your sending strategy
Gradual IP warming: Follow your ESP's recommendations to slowly increase sending volume.
Content consistency: Avoid major design or subject line changes immediately after migration.
Active list segmentation: Send to your most engaged subscribers first to build positive reputation.
Clear unsubscribe: Make it easy for recipients to opt out, reducing spam complaints.
Views from the trenches
Best practices
Always follow your new ESP's IP warming guidelines meticulously, even if it feels slow.
Thoroughly clean your email list before migrating, removing all known invalid or unengaged contacts.
Verify all email authentication records (SPF, DKIM, DMARC) are correctly configured and aligned.
Monitor granular deliverability metrics, including bounces, complaints, and blocklists, on an ongoing basis.
Maintain consistency in content, subject lines, and sending frequency immediately after migration.
Common pitfalls
Skipping or rushing the IP warming process can quickly damage your new sender reputation.
Migrating an uncleaned email list, especially one with old or purchased addresses, leads to spam traps.
Assuming deliverability will automatically improve just by switching ESPs, without addressing underlying issues.
Making drastic changes to email content or sending volume immediately after the migration.
Expert tips
Leverage DMARC reports to identify authentication failures and sources of unauthorized sending.
Regularly check your domain and IP on major public blocklists (blacklists) to catch potential issues early.
Utilize Google Postmaster Tools and other ISP-specific feedback loops for detailed performance insights.
Communicate proactively with your new ESP's support team for insights into their platform's deliverability.
Consider a phased migration approach, sending smaller volumes through the new ESP while phasing out the old.
Marketer view
The narrative suggests that deliverability often falls when moving platforms and can take three to six months to recover.
2019-09-18 - Email Geeks
Marketer view
It is possible that the "delivered" percentage decrease is due to how the numbers are calculated, rather than an actual drop in accepted mail.
2019-09-18 - Email Geeks
Restoring your email performance
Diagnosing email deliverability drops after an ESP migration involves a holistic review of technical setup, sender reputation, list hygiene, and sending practices. While an initial dip is common due to IP warming, a sustained decline points to deeper issues. By proactively monitoring key metrics, ensuring proper email authentication, cleaning your lists, and adhering to best practices, you can effectively troubleshoot and restore your email performance. The key is vigilance and a willingness to adjust your strategy based on the data.
Gmail addresses, it suggests a targeted issue with that particular Mailbox Provider.
