What KPIs should I use to monitor email deliverability trends?
Michael Ko
Co-founder & CEO, Suped
Published 13 May 2025
Updated 19 Aug 2025
9 min read
Monitoring email deliverability effectively requires more than just glancing at basic campaign reports. To truly understand whether your emails are consistently reaching the inbox and to proactively identify potential issues, you need to track specific key performance indicators (KPIs) over time. Focusing on trends, rather than isolated daily numbers, provides a clearer picture of your sending health.
Many marketers primarily look at open rates, but with privacy changes like Apple Mail Privacy Protection, this metric has become less reliable for gauging true inbox placement. While open rates still offer some directional insight, especially when viewed as a trend, relying solely on them can be misleading.
The key is to combine engagement metrics with deliverability-specific data points. This approach helps you detect subtle shifts in recipient engagement and identify any red flags that could lead to your emails being blocked or routed to the spam folder. Building a comprehensive view of your email program's health is essential for sustained success.
Core deliverability KPIs
The foundation of deliverability monitoring rests on several core KPIs that directly indicate whether your emails are reaching their intended recipients. These metrics are often available through your email service provider (ESP) or dedicated deliverability tools.
One of the most critical KPIs is the bounce rate. A high bounce rate, especially an increase in hard bounces, signals problems with your mailing list quality. It can also indicate that Internet Service Providers (ISPs) are rejecting your emails. Paying close attention to the specific bounce reasons provided in your logs is crucial, as they often offer clear pathways for remediation. A sudden spike in bounces, for instance, might point to an issue with a particular domain or a broader reputation problem.
Another vital metric is the complaint rate. This measures the percentage of recipients who mark your email as spam. High complaint rates are a strong indicator of low engagement or irrelevant content, and they can severely damage your sender reputation. A sudden drop in complaints, counter-intuitively, can sometimes signal that your emails are being bulked (sent to spam) before recipients even have a chance to complain. This highlights the importance of looking at multiple metrics in conjunction.
Bounce rate: Percentage of emails that could not be delivered.
Complaint rate: Percentage of recipients who marked your email as spam.
Blocklist status: Whether your IP or domain is listed on any major blocklists.
Inbox placement rate: The percentage of emails that successfully land in the primary inbox, not spam or promotions. This can be challenging to measure accurately without specialized tools or seed list testing.
While campaign-level metrics provide some insight, they don't always show the full picture of where your emails are landing. For a more precise understanding, you need to track inbox placement rate. This involves using seed lists or dedicated deliverability testing tools to see exactly where your emails land across various ISPs. A consistent decline in inbox placement, even if other metrics seem stable, is a strong indicator of a looming deliverability problem.
Additionally, DMARC reports are invaluable for monitoring authentication issues and identifying unauthorized sending from your domain. These XML reports provide detailed insights into email authentication results (SPF, DKIM, DMARC alignment) from major receivers like Gmail and Yahoo. A sudden drop in DMARC compliance could indicate a configuration error or even a spoofing attempt against your domain. Understanding how DMARC, SPF, and DKIM work is fundamental to leveraging these reports effectively.
Leveraging ISP feedback loops and postmaster tools
Beyond the metrics available in your ESP, directly accessing data from ISPs through their postmaster tools and feedback loops is crucial. These platforms offer unique insights into how each major mailbox provider views your sending reputation. Google Postmaster Tools, for example, provides detailed dashboards for spam rate, IP reputation, domain reputation, authentication errors, and delivery errors. Regularly checking these dashboards allows you to spot trends or sudden drops in reputation that might not be immediately apparent from your internal metrics.
Microsoft's Smart Network Data Services (SNDS) provides similar data for Outlook and Microsoft properties. While SNDS might have its quirks, it's still a valuable source for understanding your sending performance with these critical recipients. Many ISPs also offer feedback loops (FBLs), which are services that send you a report when a recipient marks your email as spam. Integrating FBL data into your monitoring strategy is essential for quickly identifying campaigns or segments that are generating high complaint rates and taking corrective action.
Positive indicators
High delivery rate (low bounces) across all campaigns.
Consistent sender reputation status in Postmaster Tools (e.g., High).
By actively monitoring these feedback loops and postmaster tools, you can get a holistic view of your sender reputation and inbox placement trends directly from the source. This data is often more reliable for deliverability assessment than click-through rates or open rates alone, as it comes directly from the mail servers themselves. Remember that even small, consistent changes in these metrics can indicate significant shifts in your deliverability over time, requiring prompt attention.
Trend analysis and segmentation
The real power in deliverability monitoring comes from analyzing these KPIs over time, rather than in isolation. A single bad day's performance might be an anomaly, but a consistent downward trend in inbox placement or an upward trend in bounce rates signals a systemic issue. Graphing metrics over weeks and months provides the necessary context to understand your true deliverability health. You simply cannot rely on one-day numbers.
Furthermore, segmenting your data by recipient ISP (e.g., Gmail, Yahoo, Outlook.com, and other domains) offers granular insights. A deliverability problem might be confined to a single ISP, indicating a specific issue with their filters or your sending practices towards their users, rather than a global reputation hit. This segmentation allows for targeted troubleshooting and optimization. For instance, if your Gmail deliverability drops but Yahoo remains stable, you can investigate Gmail-specific factors. This detailed view is essential for identifying the cause of declining deliverability.
By establishing baselines and benchmarks, you can quickly identify when a KPI deviates significantly from its normal range. For example, if your average complaint rate for Gmail users is typically 0.05%, and it jumps to 0.15% for a sustained period, that's a clear signal to investigate. Remember, the goal is not just to collect data, but to use it to inform your sending strategy and proactively address any issues that arise. You can also compare your performance to industry benchmarks to see how you stack up against other senders.
Beyond the numbers: content and audience
While technical KPIs are paramount, email deliverability is also heavily influenced by the quality and relevance of your content and the engagement of your audience. ISPs (including Google and Yahoo, with their new sender requirements) increasingly use recipient engagement as a strong signal for inbox placement. A high click-through rate (CTR) and click-to-open rate (CTOR), alongside low unsubscribe rates, indicate that your subscribers value your emails. These positive interactions can bolster your sender reputation, even if they don't directly measure deliverability.
Regularly cleaning your email list to remove inactive or unengaged subscribers also plays a critical role. Sending emails to disengaged users can artificially inflate your bounce rates and increase the likelihood of spam complaints or hitting spam traps, both of which negatively impact deliverability. Focus on maintaining a healthy, engaged list, as this is a fundamental best practice for long-term inbox success.
Ultimately, monitoring deliverability trends is an ongoing process that combines technical metric analysis with an understanding of audience engagement. By leveraging a diverse set of KPIs and breaking them down by ISP and over time, you can gain a granular view of your email program's health, troubleshoot issues effectively, and ensure your messages consistently reach the inbox. This proactive approach is key to maintaining a strong sender reputation and achieving your email marketing goals. Learning why your emails are going to spam is often the first step in this journey.
Views from the trenches
Best practices
Always graph your email deliverability metrics over time to identify trends.
Segment your deliverability data by recipient ISP (e.g., Gmail, Yahoo, Microsoft) for targeted analysis.
Regularly check major blocklists (blacklists) to ensure your sending IPs and domains are clean.
Common pitfalls
Relying solely on one-day email deliverability numbers rather than looking at long-term trends.
Ignoring bounce logs and the specific reasons for rejections, missing opportunities for remediation.
Overlooking a sudden drop in complaint rates as a positive sign, when it could indicate bulking.
Expert tips
If you see a sudden drop in complaints, it's often an indicator that your emails are being bulked, meaning they are going directly to spam.
The adoption of Apple's privacy changes is not yet at 100%, so open rate trends can still offer some directional insight, but combine them with other metrics.
Consider how your email metrics align with overall business KPIs like revenue; if email-driven revenue trends differently, it warrants investigation.
Marketer view
A marketer from Email Geeks says that historically, open rates at a domain level provided a general sense of how things were performing, though Apple's recent changes have complicated this metric's reliability.
2021-09-29 - Email Geeks
Marketer view
A marketer from Email Geeks says that it is essential to graph all metrics over time and focus on trends, because relying on single-day numbers can be misleading.
2021-09-29 - Email Geeks
Continuous improvement and adaptation
The landscape of email deliverability is constantly evolving, with ISPs regularly updating their filtering algorithms and policies. This dynamic environment necessitates a proactive and adaptive approach to monitoring. What works today might not work tomorrow, making continuous vigilance and adjustment of your strategy essential. Understanding email deliverability issues is an ongoing process.
By consistently tracking the right KPIs, analyzing trends, and segmenting your data, you are well-equipped to detect deliverability shifts early and implement corrective actions. This diligent monitoring protects your sender reputation, maximizes your email campaign performance, and ensures your valuable messages consistently reach your audience's inbox, fostering strong engagement and driving business results.
