How can I effectively use SparkPost's delivery index and other KPIs to measure and improve email marketing health and deliverability?
Michael Ko
Co-founder & CEO, Suped
Published 3 May 2025
Updated 19 Aug 2025
8 min read
Understanding email deliverability and marketing health is a complex challenge. Many factors influence whether your emails land in the inbox, a spam folder, or simply disappear. While various metrics offer glimpses into performance, getting a comprehensive picture requires looking beyond a single indicator.
Email service providers (ESPs) like SparkPost provide their own tools and metrics, such as the Delivery Index, to help users gauge their performance. The key is to know how to interpret these tools alongside other vital Key Performance Indicators (KPIs) to truly measure and improve your email marketing strategy.
It is important to remember that no single metric provides a complete view of your email program's health. A holistic approach, combining various data points, is essential for accurate assessment and effective optimization.
Understanding SparkPost's delivery index
The SparkPost Delivery Index is designed to offer a quick snapshot of your email program's overall health and deliverability. It aggregates various data points to give you a single score, acting as a high-level indicator. Think of it as a health monitor for your email sending, much like a fitness tracker for your physical well-being. It aims to tell you if your emails are reaching the inbox or falling into spam or junk folders.
While it can be a useful tool for identifying general trends and potential issues, it is not a standalone metric to optimize directly. As with any aggregated score, it provides an overview, but the underlying components are where the real actionable insights lie. A high index suggests good health, but a sudden drop warrants a deeper investigation into specific metrics that contribute to it.
It is important to note that different Internet Service Providers (ISPs) have varying behaviors regarding content and mail stream reputation. A single measure like the Delivery Index might not perfectly reflect performance across all ISPs. Therefore, while SparkPost’s data is generally reliable, always cross-reference with other indicators and break down your data by recipient ISP for a more granular understanding.
What the delivery index tells you
The SparkPost Delivery Index primarily indicates whether your messages are reaching the inbox or are being filtered into spam. It accounts for various factors affecting inbox placement, offering a simplified score for easy monitoring. While it can show correlation with your sending practices, it's a diagnostic tool rather than a target to optimize directly.
Key deliverability KPIs
Beyond the aggregated scores, a suite of specific Key Performance Indicators (KPIs) offers deeper insights into your email marketing health and deliverability. These metrics allow you to pinpoint exact areas for improvement and understand the nuances of your sending reputation. Focusing on these granular details is crucial for long-term success.
The most fundamental KPIs include open rates, click-through rates (CTR), bounce rates, and spam complaint rates. Changes in unique opens and clicks, when smoothed over several mailings, are particularly meaningful. For instance, if your open rates drop significantly while unique clicks remain stable, it could indicate an issue with inbox placement rather than content engagement. This specific scenario points towards a deliverability problem that needs investigation, perhaps by breaking down data by recipient ISP.
Tracking these metrics allows you to identify trends that may not be immediately apparent in a consolidated index. For example, a sudden spike in bounce rates after a list segmentation could suggest issues with list hygiene. Similarly, an increase in spam complaints could point to content relevance issues or an audience that is not engaged. Monitoring KPI trends over time provides a much clearer picture of your email program's trajectory.
To effectively use email deliverability monitoring tools, integrating data from your ESP with web analytics is a best practice. This allows for a complete view, from email delivery to conversion on your website, providing an end-to-end understanding of your email marketing impact.
Improving email marketing health
Once you have a clear understanding of your KPIs, the next step is to use this data to actively improve your email marketing health. Analyzing trends and identifying anomalies is crucial for this process. For example, a consistent decline in open rates might suggest a sender reputation issue or content fatigue among your subscribers.
A proactive approach includes regular list hygiene to remove inactive or invalid email addresses, which can lead to hard bounces and negatively impact your sender score. Implementing clear consent processes for new subscribers can also help reduce spam complaints. Personalizing content and sending at optimal times based on user engagement data can also significantly boost your metrics.
Furthermore, a sudden increase in bounces or complaints could signal that your domain or IP has been placed on a blocklist or blacklist. Regularly checking these lists is vital. If listed, prompt action is required to remediate the issue and request removal. Continuous monitoring and adaptation are key to maintaining strong email deliverability.
Identifying the problem
Declining open rates: Messages are likely not reaching the inbox, or content is losing relevance. This may point to a reputation issue.
High bounce rate: Sending to invalid or non-existent email addresses. This negatively impacts your sender score and could lead to blocklisting (or blacklisting).
Increased spam complaints: Recipients marking your emails as spam, indicating content irrelevance or unsolicited mail.
Implementing solutions
Content optimization: Refine email content to be more engaging and relevant to your audience.
List hygiene: Regularly clean your email lists to remove inactive or invalid addresses, reducing bounce rates.
Feedback loops: Implement ISP feedback loops to quickly remove users who complain, preserving sender reputation.
By consistently analyzing these core KPIs and implementing targeted solutions, you can significantly improve your email marketing health and ensure your messages consistently reach their intended audience. It's an ongoing process that requires attention and adaptation.
Technical indicators and reputation
Beyond engagement metrics, the underlying technical configuration of your email sending infrastructure plays a pivotal role in deliverability. Proper implementation of email authentication protocols, such as SPF, DKIM, and DMARC, is non-negotiable for modern email marketing. These protocols verify your sender identity and protect your domain from spoofing and phishing attempts, which directly impacts your sender reputation.
SparkPost and other ESPs offer analytics that include authentication pass/fail rates. Monitoring these closely can help you identify configuration errors. For example, a high rate of DKIM failures could indicate an issue with your DNS record. Ensuring that your DMARC, SPF, and DKIM records are correctly set up and aligned is fundamental for establishing trust with receiving mail servers.
Your sender reputation is also heavily influenced by how your IP addresses and domains are perceived across the internet. Being listed on a DNS-based blocklist (or blacklist) can severely impact deliverability. These lists track IPs and domains involved in sending spam or malicious content. Regularly performing a blacklist check and proactively monitoring your sender reputation is vital. If you find your domain or IP on a blocklist (or blacklist), immediate steps to delist yourself are necessary to restore your email deliverability. This includes identifying the root cause of the listing, resolving it, and then following the specific delisting procedures for each blocklist (or blacklist).
Moreover, dedicating IP addresses (if your sending volume warrants it) and managing their reputation through consistent, high-quality sending is another advanced strategy. Monitoring your sender score with tools like Google Postmaster Tools also provides valuable insights into how major ISPs view your sending practices. A comprehensive approach to these technical indicators is foundational for robust deliverability.
Views from the trenches
The experiences of email professionals often highlight the nuances of using various metrics. Here are some insights from the community.
Best practices
Regularly segment your audience to send more targeted and relevant content, which improves engagement and sender reputation.
Always use a double opt-in process for new subscribers to ensure high-quality, engaged email lists from the start.
Monitor soft and hard bounce rates diligently and remove hard-bouncing addresses immediately to maintain list hygiene.
Ensure SPF, DKIM, and DMARC records are correctly configured and aligned to build trust with receiving mail servers.
Common pitfalls
Over-reliance on a single aggregated metric like a 'Delivery Index' without diving into the underlying KPIs can mask specific problems.
Ignoring feedback loops from ISPs, which can lead to continued sending to recipients who have marked your emails as spam.
Failing to regularly clean email lists, resulting in high bounce rates and a degraded sender reputation.
Sending inconsistent email volumes or sudden large blasts from a new IP, which can trigger spam filters and blocklists.
Expert tips
Break down deliverability data by recipient ISP to identify provider-specific issues that require tailored solutions.
Focus on the trend of engagement metrics like unique opens and clicks over time rather than isolated campaign performance.
Implement a DMARC policy with reporting to gain visibility into your email authentication status and potential spoofing attempts.
Continuously test email content and subject lines to optimize engagement and avoid spam triggers.
Expert view
Expert from Email Geeks says a single measure like SparkPost's Delivery Index won't match all different recipient ISP behaviors because they vary widely regarding content and mail stream reputation.
2020-02-21 - Email Geeks
Expert view
Expert from Email Geeks says the SparkPost Delivery Index is probably a useful tool to spot potential areas for improvement, but optimizing for it directly would likely be a mistake.
2020-02-21 - Email Geeks
These insights underscore the importance of a nuanced approach to email deliverability. While aggregated scores offer a quick view, granular analysis and continuous monitoring of various KPIs are essential for identifying specific issues and driving improvement.
The path to better deliverability
Effectively using tools like SparkPost's Delivery Index, combined with a deep dive into individual KPIs, is critical for understanding and enhancing your email marketing health. While the Delivery Index offers a helpful high-level overview, it is the specific metrics like open rates, click-through rates, bounce rates, and spam complaints that provide actionable insights.
A holistic strategy involves not only monitoring these numbers but also consistently implementing best practices in list management, content creation, and email authentication. Proactive monitoring of blocklists (or blacklists) and swift action to address any issues are equally important for maintaining a strong sender reputation and ensuring your emails reach the inbox.
Ultimately, the goal is to drive business value through email. By integrating data, maintaining vigilance, and adapting your strategies based on comprehensive KPI analysis, you can significantly improve your email deliverability and achieve greater success with your marketing campaigns.
SparkPost provide their own tools and metrics, such as the Delivery Index, to help users gauge their performance. The key is to know how to interpret these tools alongside other vital Key Performance Indicators (KPIs) to truly measure and improve your email marketing strategy.
