How should I interpret discrepancies between Return Path/DeliveryIndex and Google Postmaster Tools data?
Matthew Whittaker
Co-founder & CTO, Suped
Published 7 Jul 2025
Updated 19 Aug 2025
8 min read
Many email senders rely on various tools to gauge their email deliverability, but it's common to see conflicting data. One common point of confusion arises when comparing data from traditional inbox placement vendors, like Return Path or DeliveryIndex, with insights from Google Postmaster Tools.It can be perplexing when one source shows high spam rates while the other indicates excellent reputation and low complaints.
This difference in reporting can lead to uncertainty about your true email performance and whether immediate action is needed. Understanding the nuances of each platform's data collection and reporting methodologies is crucial to accurately interpret what the numbers mean for your email program. We will explore why these discrepancies happen and how to make sense of the combined information.
How data collection differs
Return Path and DeliveryIndex typically operate using seed lists and panel data. They send emails to a network of email addresses (seeds) across various internet service providers (ISPs) and then track where those emails land, in the inbox, spam folder, or if they are blocked (blacklisted or blocklisted). This approach aims to provide a broad overview of inbox placement across a diverse set of mailboxes.
The challenge with panel data is its representativeness. If your audience is heavily skewed towards one ISP, say Gmail, and the panel data has a smaller proportion of those specific mailboxes, the insights might not accurately reflect your actual performance with that dominant ISP. Mailbox providers are also becoming more sophisticated at detecting and filtering seed list traffic.
Google Postmaster Tools (GPT), on the other hand, provides data directly from Google's own internal systems. This means it offers a highly accurate view of how your emails are performing specifically with Gmail users. GPT provides insights into various metrics including spam rate, IP reputation, domain reputation, feedback loop (FBL) data, and delivery errors, but exclusively for emails sent to Gmail recipients. You can learn more about GPT in this Google Workspace Admin help article.
Why your data might not align
The primary reason for discrepancies lies in the different data sources and methodologies. Return Path and DeliveryIndex rely on external seed networks, which provide a generalized snapshot. This snapshot may not perfectly reflect the experience of your actual subscribers, especially if your recipient list demographics do not match the seed list's composition. For instance, if your list is primarily Gmail users, Return Path's data, which might be influenced by Comcast or other ISPs, will not directly correlate. Additionally, the methods used by Return Path and DeliveryIndex are becoming increasingly unreliable as ISPs crack down on bot behavior.
Google Postmaster Tools reports on data directly observed by Gmail. However, it does not provide 100% of all complaint data. As some deliverability experts have noted, Gmail's feedback loop system often provides samples rather than a comprehensive total of all complaints, leading to lower reported spam complaint rates compared to what might be observed elsewhere or internally. This is why you might see a 0.0% spam feedback loop in GPT even if you suspect otherwise. You can read more about how to interpret GPT's data on SparkPost's website.
Another factor is the distinction between "spam complaints" and "spam placement percentage." A high spam placement percentage (emails landing in the spam folder) reported by a tool like Return Path doesn't necessarily mean a high complaint rate in GPT. In fact, if emails are heavily filtered to spam, fewer recipients will see them and fewer will mark them as spam, leading to a potentially lower observed complaint rate in Google Postmaster Tools. This is a crucial difference to understand when comparing reports.
Return Path/DeliveryIndex
Data source: Relies on third-party seed lists and panel data.
Coverage: Provides broad insights across multiple ISPs.
Metrics: Often focuses on inbox placement and blocklist status.
Accuracy: Can be less precise for specific ISPs, especially Gmail, due to sampling.
Google Postmaster Tools
Data source: Direct data from Google's internal systems.
Coverage: Exclusively for Gmail recipients.
Metrics: Focuses on spam rate, IP/domain reputation, FBLs, and delivery errors.
Accuracy: Highly accurate for Gmail performance, but may not report all complaint data.
Prioritizing and interpreting your data
When faced with conflicting data, it's best to prioritize the metrics that directly impact your email program and align with your audience. If a significant portion of your subscribers are Gmail users, then Google Postmaster Tools data should be your primary source for Gmail-specific performance. It gives you the direct insight from the mailbox provider itself, making it invaluable for understanding your reputation with Google.
If your open rates across different ISPs are relatively consistent, even with minor fluctuations, it often suggests that you do not have severe, widespread deliverability issues. For example, if campaigns reported with a 30-40% spam rate by Return Path still show open rates only 2-3% lower than campaign with <10% spam rate, it implies a more nuanced problem than a complete inbox failure. You can dive deeper into which metrics to focus on in this article: What email deliverability metrics to monitor.
Remember that Return Path and DeliveryIndex data, while providing a general health check, are based on sampled data that may not always reflect your unique sending patterns or recipient engagement. Focus on identifying trends rather than reacting to every single data point from these tools. For common issues with GPT data, read What are common issues and outages with GPT data?.
Key considerations for data interpretation
Audience composition: If your list is predominantly Gmail users, prioritize Google Postmaster Tools for specific deliverability insights.
Metric definitions: Understand whether a tool reports on spam complaints (explicit recipient action) or spam placement (emails landing in the spam folder). These are distinct and will show different numbers.
Engagement signals: Always cross-reference with your internal metrics like open rates, click-through rates, and conversion rates, as these reflect actual recipient behavior and can be highly indicative of inbox placement.
Actionable steps for clarity
To gain clearer insights, focus on comparing similar metrics across platforms and understanding their limitations. If Google Postmaster Tools shows a "Bad" or "Low" domain reputation, this is a clear signal from Gmail that warrants investigation, regardless of other tools. Conversely, if your GPT reputation is "High" with low spam rates, but a third-party tool indicates high spam, consider the possibility that the third-party tool's seed list is not representative of your actual Gmail audience.
Regular monitoring of both data sources is essential. Use GPT to troubleshoot specific Gmail deliverability issues, like a sudden drop in IP reputation or an increase in authentication failures. For example, if you see inaccurate SPF and DKIM authentication rates, you can refer to our guide on why GPT shows incorrect SPF and DKIM rates. Implementing robust email authentication standards like SPF, DKIM, and DMARC is critical for building sender trust and improving deliverability across all mailbox providers.
Configuring these records correctly is fundamental. Discrepancies in authentication results between tools can occur, so knowing which data to prioritize is important. You can find more information on DMARC monitoring to ensure your configuration is optimal. Continual observation of your sending practices and engagement metrics, alongside these tools, provides the most comprehensive picture.
Always prioritize your internal engagement metrics like open and click-through rates.
Use Google Postmaster Tools as your primary source for understanding Gmail-specific deliverability.
Consistently monitor your domain and IP reputation in GPT, as these are strong indicators.
Common pitfalls
Over-relying on third-party seed list data without considering your specific audience.
Confusing spam placement rates (emails going to spam) with explicit spam complaint rates.
Expecting Google Postmaster Tools to report 100% of all spam complaints.
Expert tips
If a high percentage of your subscriber list is Gmail, ensure your email authentication is configured.
Analyze trends over time rather than reacting to daily fluctuations in external tool reports.
Actively manage unengaged subscribers to reduce spam complaints and improve list health.
Marketer view
Marketer from Email Geeks says that their Postmaster Tool showed a 0.0% spam feedback loop, which did not align with their observations.
2020-02-27 - Email Geeks
Marketer view
Marketer from Email Geeks says that Gmail feedback loops do not show 100% of complaints and they do not have a fully functional feedback loop system.
2020-02-27 - Email Geeks
Navigating your deliverability data
In the complex world of email deliverability, relying on a single data source can be misleading. Discrepancies between Return Path/DeliveryIndex and Google Postmaster Tools are not uncommon and stem from their fundamental differences in data collection. Return Path provides a broader, sampled view across various ISPs, while Google Postmaster Tools offers deep, direct insights specifically for Gmail.
To effectively manage your email program, integrate insights from all available tools. Prioritize Google Postmaster Tools for Gmail performance and your internal engagement metrics (opens, clicks, unsubscribes) for the most accurate picture of your audience's interaction. Use third-party tools as supplementary indicators for general trends, always being mindful of their methodologies and potential biases.
Gmail, and the panel data has a smaller proportion of those specific mailboxes, the insights might not accurately reflect your actual performance with that dominant ISP. Mailbox providers are also becoming more sophisticated at detecting and filtering seed list traffic.
Google Postmaster Tools (GPT), on the other hand, provides data directly from Google's own internal systems. This means it offers a highly accurate view of how your emails are performing specifically with Gmail users. GPT provides insights into various metrics including spam rate, IP reputation, domain reputation, feedback loop (FBL) data, and delivery errors, but exclusively for emails sent to Gmail recipients. You can learn more about GPT in this Google Workspace Admin help article.
Comcast or other ISPs, will not directly correlate. Additionally, the methods used by Return Path and DeliveryIndex are becoming increasingly unreliable as ISPs crack down on bot behavior.
DMARC is critical for building sender trust and improving deliverability across all mailbox providers.
