How reliable is ReturnPath inbox rate and why might it not correlate with open rates?
Michael Ko
Co-founder & CEO, Suped
Published 7 Aug 2025
Updated 19 Aug 2025
7 min read
When comparing the inbox rate reported by a third-party service like Return Path with my own open rates, I've observed a common scenario: the open rate often remains consistent, perhaps in the 12-13% range, regardless of what Return Path reports for inbox placement. This raises a crucial question about the reliability of such inbox rate metrics and why they might not align with a sender's actual open rates.
The disconnect between reported inbox rates and actual open rates is a topic that frequently comes up in discussions among email professionals. Understanding why this happens requires a deeper dive into how inbox placement is measured, how open rates are tracked, and the inherent limitations of both metrics in the evolving email landscape.
The nuances of inbox rate reporting
Third-party inbox placement monitoring services, including Return Path (now part of Validity), rely on seed lists. These are special email addresses distributed across various internet service providers (ISPs) and spam filters. When you send a campaign, a copy is also sent to these seed addresses, and the service then reports where those emails landed: inbox, spam, or missing.
While useful, this methodology provides an estimate. The composition of the seed list (which ISPs are included and how they are weighted) might not perfectly reflect the demographics of your actual subscriber list. If, for instance, a seed list over-represents an ISP where you generally have poor placement, the reported inbox rate could appear lower than your overall actual deliverability, or vice versa if it under-represents a strong performing ISP. As some experts note, any third-party inbox rate can be unreliable to varying degrees.
Furthermore, a service's reported inbox rate might be influenced by broader industry trends that don't directly apply to your specific sending practices or audience. For example, the average global inbox placement rate can fluctuate over time. In 2013, the average was around 76.5% globally, but this can vary significantly by region and year.
How seed lists work
Seed lists are a network of email addresses at various email providers. When you send a campaign, a copy is sent to these addresses to simulate delivery and determine inbox placement. This gives an indication of how your emails are performing across different ISPs and spam filters.
Limitations of seed lists
ISP weighting: The distribution of email addresses in a seed list might not match your actual subscriber list's ISP breakdown, leading to skewed results.
Dynamic filtering: ISP filtering is highly personalized and dynamic. A seed list may not fully replicate how individual subscriber engagement impacts placement.
Engagement signals: Seed lists cannot replicate user engagement (opens, clicks, replies), which is a significant factor for inbox placement at major providers like Google and Outlook.
Metric
Description
Reliability
Return Path Inbox Rate
Estimates where emails land using a seed list, indicating successful delivery to the inbox versus spam folder.
Provides an industry benchmark, but may not precisely reflect your list due to seed list composition and weighting.
Open Rate
Measures the percentage of recipients who opened your email, typically by loading an invisible tracking pixel.
Historically a key metric, but its accuracy is now compromised by privacy features and bot activity, making it less reliable.
The evolving landscape of open rates
For years, open rates were considered a primary indicator of email engagement and, by extension, inbox placement. If your open rates were high, it often implied your emails were landing in the inbox and resonating with subscribers. However, the landscape has changed significantly, making open rates unreliable as a standalone metric.
The introduction of privacy features like Apple Mail Privacy Protection (MPP) has fundamentally altered how opens are tracked. MPP preloads images, including tracking pixels, for users of Apple Mail, regardless of whether the user actually views the email. This can artificially inflate open rates, making it difficult to discern genuine engagement from automated activity. You can learn more about how Gmail image caching affects accuracy.
Beyond privacy features, email bots and security scanners also contribute to inaccurate open rates by triggering tracking pixels without human interaction. This means a high open rate might simply indicate strong bot activity rather than successful inbox placement or user interest. Therefore, relying solely on open rates can lead to misleading conclusions about email campaign performance and deliverability.
Open rates are not what they used to be
With the rise of privacy features like Apple Mail Privacy Protection, email open rates are no longer a reliable indicator of genuine engagement. These features often pre-fetch images, triggering open pixels regardless of whether the recipient actually views the email.
Old understanding
A high open rate meant emails landed in the inbox and users were actively reading them. It was a primary metric for campaign success and deliverability.
New reality
Open rates are often inflated by privacy features and bot activity. While still tracked, they need to be interpreted cautiously and supported by other engagement metrics like clicks and conversions. You may also notice a sudden drop in email open rates.
Why discrepancies occur
The fundamental reason why Return Path's inbox rate might not correlate with your open rates lies in the different ways these metrics are generated and what they truly represent. An inbox placement tool measures where an email lands for a seed list, which is a synthetic representation of the broader email ecosystem. Open rates, on the other hand, reflect interactions (both human and automated) from your actual subscriber list.
It's possible to have a consistent open rate even if your overall inbox placement varies. This can happen if a significant portion of your subscriber list consists of highly engaged individuals who consistently open your emails, regardless of whether they land in the primary inbox or a secondary tab (like Gmail's Promotions tab). These dedicated openers can stabilize your observed open rate, even if a larger segment of your list experiences deliverability challenges.
Another factor is the personalized nature of inbox placement. ISPs use a complex array of signals, including historical engagement, sender reputation, and content, to decide where an email lands. Your overall sender reputation, including factors like your email domain reputation, plays a significant role. Even if a seed list reports a lower inbox rate due to generalized issues, your strong relationship with certain subscribers can override these signals for them, ensuring your messages consistently reach their inbox.
Given the limitations of single metrics, it's essential to adopt a holistic approach to understanding and improving email deliverability. Instead of focusing solely on Return Path's reported inbox rate or your basic open rates, look at a broader set of indicators. Click-through rates (CTR) and conversions are far more reliable metrics for measuring actual engagement and campaign performance.
It's also crucial to monitor your sender reputation proactively. This involves keeping an eye on your email domain reputation, tracking spam complaint rates, and utilizing tools like Google Postmaster Tools for direct insights from major ISPs. Maintaining a clean email list by regularly removing unengaged subscribers and invalid addresses is also key to preventing your emails from ending up on a blacklist or blocklist.
Focus on email authentication (SPF, DKIM, DMARC) and sending relevant, valuable content. These foundational elements build trust with ISPs and subscribers, which ultimately has a greater impact on your long-term deliverability than any single reported metric. Remember, true email deliverability means your messages reach the intended recipient's inbox and are engaged with, not just that they avoided a spam folder in a test.
Prioritize genuine engagement
While inbox rate reports offer clues, the ultimate goal is subscriber engagement. Focus on metrics like click-through rate, conversion rate, and replies, as these truly indicate that your emails are not only reaching the inbox but also resonating with your audience.
Monitor core deliverability factors
Sender reputation: Regularly check your IP and domain reputation, as a poor reputation can lead to emails being blocked or sent to spam.
Spam complaints: High complaint rates are a strong negative signal. Work to reduce them by improving list quality and content relevance.
List hygiene: Regularly clean your email list to remove inactive or invalid addresses, which can improve overall deliverability and reduce bounce rates.
Views from the trenches
Best practices
Always consider your audience's actual ISP distribution against any seed list's weighting to properly interpret inbox rates.
Focus on engagement metrics like click-through rates and conversions as more accurate indicators of campaign success than open rates.
Common pitfalls
Solely relying on a single third-party inbox rate report without cross-referencing with other data sources.
Interpreting static open rates as proof of good deliverability, ignoring the impact of privacy features and bots.
Expert tips
An expert from Email Geeks suggests that the personalization of inbox placement means an engaged segment can pull emails into the inbox even if overall reputation is fluctuating.
An expert from Email Geeks points out that if your core audience consistently opens emails, their engagement can keep your open rates stable, masking potential inboxing issues for less engaged recipients.
Expert view
Expert from Email Geeks says that any third-party-reported inbox rate is unreliable to varying degrees, as their methodology may not align with a sender's specific audience profile.
2020-03-02 - Email Geeks
Expert view
Expert from Email Geeks notes that differing ISP weighting in a third-party tool's seed list compared to a sender's actual list can significantly skew reported inbox rates.
2020-03-02 - Email Geeks
A comprehensive approach to email success
The correlation (or lack thereof) between Return Path's reported inbox rate and your open rates stems from the inherent differences in how these metrics are generated and what they truly measure. While services like Return Path offer valuable insights through seed list testing, they provide an estimated view of inbox placement that might not perfectly align with the complex, personalized filtering applied to your live subscriber list.
The evolving nature of open rate tracking, influenced by privacy features and bot activity, further complicates the picture. Ultimately, a robust email strategy moves beyond relying on a single metric. By focusing on a combination of reliable engagement metrics, proactive reputation management, and strong authentication, you can gain a more accurate understanding of your deliverability and achieve true email marketing success.
Google and 
Outlook.
Apple Mail Privacy Protection (MPP) has fundamentally altered how opens are tracked. MPP preloads images, including tracking pixels, for users of Apple Mail, regardless of whether the user actually views the email. This can artificially inflate open rates, making it difficult to discern genuine engagement from automated activity. You can learn more about how Gmail image caching affects accuracy.
Gmail's Promotions tab). These dedicated openers can stabilize your observed open rate, even if a larger segment of your list experiences deliverability challenges.
