Why does a new ESP perform worse than an old ESP despite similar deliverability metrics?
Michael Ko
Co-founder & CEO, Suped
Published 9 May 2025
Updated 19 Aug 2025
8 min read
Moving to a new Email Service Provider (ESP) can be a complex process, often accompanied by the expectation that existing deliverability performance will seamlessly transfer. It's frustrating when you see your inbox placement metrics, like those from email deliverability tests, remain high, yet your engagement metrics, such as clicks and sign-ups, plummet, especially for large-scale marketing sends (often called "blasts"). This discrepancy can be puzzling when smaller, triggered emails continue to perform well.
I've encountered this scenario many times. The challenge lies in understanding that "deliverability" isn't a single, uniform metric. Mailbox providers assess numerous factors beyond simple inbox placement, and a new ESP, even with warmed IP addresses, starts with a different trust profile. We need to dig deeper into why a seemingly healthy setup can still lead to diminished results.
The nuances of reputation beyond basic metrics
One of the most significant reasons for this performance gap lies in the nuanced nature of sender reputation. While your IP addresses might be warmed, and your basic inbox rates look good, an older ESP typically benefits from a long-standing, cumulative reputation with various internet service providers (ISPs) and mailbox providers. This includes domain reputation, which builds over years of consistent, positive sending behavior.
When you switch to a new ESP, even if you keep your sending domain, the underlying infrastructure, including the specific IP ranges and how the ESP handles mail routing and authentication, is new to the recipient systems. This means that while emails might not be outright rejected or sent to the spam folder, they might experience subtle filtering that affects visibility. This could include placement in less prominent folders, such as Gmail's Promotions tab, rather than the primary inbox. Recipients are less likely to see and interact with emails buried in these secondary folders, leading to lower clicks and conversions.
Mailbox providers maintain complex internal scoring systems for senders, often referred to as a sender score. This score is dynamic and influenced by a multitude of factors, not just whether an email makes it to the inbox. It encompasses user engagement (opens, clicks, replies), complaint rates, spam trap hits, and historical sending patterns. Even if your initial IP warming process is successful in achieving high delivery rates, the new ESP's platform-level reputation and the lack of long-term history with your domain on their specific infrastructure can still result in lower engagement.
Best practice: nurture your sender reputation
When migrating to a new ESP, focus on building and maintaining a strong sender reputation from day one. This goes beyond simply warming IPs and involves meticulous list hygiene, engaging content, and monitoring advanced metrics. Regularly check Google Postmaster Tools and Microsoft SNDS for insights into your domain and IP reputation.
Another factor is how different ESPs might handle underlying technical aspects that influence how an email is perceived by an ISP. While most ESPs provide a healthy platform, subtle variations in their email authentication setup (SPF, DKIM, DMARC), header structure, or even the underlying network configuration can lead to different treatment by recipient servers. Even if your authentication records are correctly configured, the trust associated with the new ESP's sending infrastructure is still being established.
Hidden factors influencing engagement
Another often overlooked aspect is the potential for subtle differences in how the new ESP handles email content and tracking. While you might be sending the exact same copy and creative, the ESP's internal rendering engine or link wrapping could subtly alter the email's perceived nature. This could trigger more aggressive filtering by ISPs that analyze content and link structures for spam indicators, even if the email isn't flagged as outright spam.
Moreover, some ESPs employ different methodologies for calculating metrics like open rates and click-through rates. While you might see "equivalent" open rates, the way these are tracked (e.g., pixel-based opens versus actual user engagement) might vary. A discrepancy in click rates, particularly a significant drop for large sends, suggests that even if emails are reaching the inbox, they might be experiencing some form of pre-filtering or a less prominent placement that reduces visibility and subsequent interaction.
The distinction between triggered emails and large "blast" campaigns is critical. Triggered emails, such as welcome series or transactional notifications, are typically highly anticipated and receive strong engagement. This consistent positive interaction helps build a robust sender reputation for that specific mail stream, often making them less susceptible to subtle filtering, even on a new ESP. Blasts, on the other hand, often go to a broader audience, which may include less engaged subscribers or even spam traps, increasing the risk of negative feedback or algorithmic down-ranking. This difference in engagement patterns can explain why a new ESP struggles with blasts but excels with triggered sends.
Old ESP behavior
Benefits from an established, historical sender reputation built over years. ISPs have a long track record of accepting and placing emails from this infrastructure in primary inboxes, leading to higher visibility and engagement even for large sends. Suppression lists and bounce handling are finely tuned.
Engagement Metrics: Consistent high open and click rates across all send types, including large marketing campaigns, due to strong trust and primary inbox placement.
New ESP behavior
Starts with a fresh, albeit positive, reputation on its specific infrastructure. Despite IP warming, it lacks the deep, granular trust for bulk mail. This can lead to subtle filtering to secondary tabs (e.g., Promotions), reducing visibility, even if inbox placement tools report high rates. Suppression rules might also be different.
Engagement Metrics: Triggered emails perform well due to high user anticipation and engagement. Large marketing sends may see lower clicks and conversions due to reduced visibility from subtle filtering.
It's also essential to consider the recipient's perspective and how they interact with different types of emails. Recipients are often more forgiving and engaged with transactional or triggered emails, which directly relate to an action they've taken. Marketing blasts, especially those sent to broader, less recently engaged segments, face much stricter scrutiny from ISPs and recipients alike.
Audience behavior and list segmentation
The fundamental difference between your triggered emails and large blasts often comes down to list hygiene and audience segmentation. Triggered sends typically go to highly engaged, recently opted-in subscribers, representing the most active segment of your audience. These users are expecting your emails, which naturally leads to higher engagement rates and positive signals to ISPs.
Conversely, large "blast" sends often target a much wider audience, which may include segments that are less engaged, older, or contain problematic email addresses. Even with rigorous list cleaning, there's a higher probability of encountering inactive users or even different types of spam traps on these larger lists. A new ESP, still building its holistic reputation, might be more sensitive to these negative signals, even if your domain reputation seems stable.
Furthermore, the old ESP might have had more sophisticated internal suppression mechanisms or unique ways of segmenting and managing subscriber lists that mitigated risks for large sends. While an A/A test aims for identical conditions, subtle variations in how the new ESP handles suppression, bounces, or even sends emails to specific domains can contribute to the observed drop in performance for high-volume campaigns.
This highlights the importance of not just looking at raw deliverability rates but also analyzing the underlying engagement patterns by recipient domain and segment. A drop in clicks across the board might indicate a general reputation issue, while a targeted drop with a specific provider could point to a unique filtering challenge with that ISP.
Send type
Impact on new ESP performance
Risk factors
Triggered emails
Often perform well due to high anticipation and engagement, contributing positively to initial reputation building.
Low risk. Users are expecting these emails, leading to fewer complaints and stronger positive signals.
Large "blast" campaigns
Higher chance of degraded performance (e.g., lower clicks, sign-ups) due to broader audience and evolving trust with new infrastructure.
Higher risk. May include less engaged segments, old addresses, or blocklist triggers, impacting sender reputation.
Views from the trenches
Best practices
Maintain rigorous list hygiene by regularly removing unengaged subscribers and invalid email addresses.
Segment your audience based on engagement levels and tailor sending volume and content accordingly.
Monitor your sender reputation across various ISPs using tools like Google Postmaster Tools.
Begin warming your new IP addresses and domain gradually, increasing volume incrementally over time.
Implement and monitor email authentication protocols like SPF, DKIM, and DMARC meticulously.
Common pitfalls
Assuming similar deliverability metrics mean identical inbox placement and engagement outcomes.
Failing to account for the new ESP's platform-level reputation and its impact on your mail streams.
Neglecting to segment your mailing list, sending large blasts to unengaged or problematic addresses.
Not engaging the new ESP's deliverability support team for troubleshooting complex issues.
Overlooking subtle technical differences in header injection or link wrapping that could affect filtering.
Expert tips
Analyze your click-through rates by domain, not just overall, to pinpoint specific receiver issues.
Understand that
IP warming
builds reputation, but
domain reputation
Expert view
Expert from Email Geeks says a drop in clicks and sign-ups after an ESP migration is a common issue, often attributed to subtle differences in suppression criteria or the new platform's reputation.
2022-06-15 - Email Geeks
Expert view
Expert from Email Geeks says if the new ESP offers critical features, engage their deliverability team immediately to resolve significant drops in downstream metrics like sign-ups, highlighting your long-term client commitment.
2022-06-16 - Email Geeks
Moving forward with a new ESP
The transition to a new ESP is rarely as straightforward as simply moving your lists and hitting send. While basic deliverability metrics might appear consistent, the deeper layers of sender reputation, ISP trust, and subtle technical differences can significantly impact how your emails are treated, especially high-volume marketing sends. The key is to look beyond superficial metrics and delve into granular data, like domain-specific performance and engagement rates, to identify the root cause.
Success on a new ESP, particularly for large campaigns, hinges on understanding that deliverability is a continuous process of building and maintaining trust. This requires meticulous attention to list hygiene, content relevance, and proactive monitoring of your sender health. It also often means collaborating closely with your new ESP's deliverability experts to fine-tune your sending strategy and address any lingering issues that might be affecting your valuable downstream metrics.
Why does a new ESP perform worse than an old ESP despite similar deliverability metrics? - Sender reputation - Email deliverability - Knowledge base - Suped
and Microsoft SNDS for insights into your domain and IP reputation.
