How should opens be used as a metric for email marketing?
Matthew Whittaker
Co-founder & CTO, Suped
Published 11 May 2025
Updated 19 Aug 2025
8 min read
Email open rates have long been a foundational metric in email marketing, offering a seemingly straightforward glimpse into how many recipients engage with our campaigns. For years, a high open rate was a clear indicator of a compelling subject line and a healthy sender reputation. It was the first hurdle a message had to clear to even have a chance at conversion.
However, the landscape of email privacy and technology has evolved significantly. With the introduction of features like Mail Privacy Protection (MPP) by Apple, the reliability of open rates as a true measure of engagement has come into question. These privacy changes pre-fetch emails and their tracking pixels, artificially inflating reported open rates. This means a significant portion of what appear to be opens are not actual human interactions.
Given these shifts, the way we perceive and use open rates must adapt. They are no longer the single, unambiguous beacon of success they once were. Instead, they require a more nuanced interpretation, serving as one piece of a larger puzzle rather than the sole arbiter of a campaign's performance.
The evolving role of email open rates
The implementation of Mail Privacy Protection, primarily by Apple, has fundamentally altered how open rates are tracked. When a recipient uses Apple Mail with MPP enabled, their device automatically pre-loads email content, including tracking pixels, regardless of whether the user actually opens or views the email. This results in proxy opens that appear as legitimate opens in our analytics, even if the email never truly captured the recipient's attention.
This inflation of reported open rates creates a deceptive sense of engagement. We might see higher open rates across our campaigns, leading us to believe our subject lines are performing exceptionally well or that our audience is highly engaged, when in reality, a significant portion of these opens are automated. This disconnect makes it challenging to accurately assess genuine recipient interest and campaign effectiveness using open rates alone.
Despite these challenges, open rates still offer a valuable, albeit broad, indicator. They can provide high-level insights into the overall success of your email delivery and the initial appeal of your subject lines. For instance, a sudden significant drop in open rates for a specific segment might signal deliverability issues or that your content isn't resonating. The shift in how we analyze these metrics is a topic discussed by Microsoft in their blog about rethinking email metrics.
Essentially, the open rate has transitioned from being a precise measure of individual engagement to a more general indicator of email program health. While it can still help us understand if our emails are landing in the inbox and if our initial messaging prompts a look, it no longer definitively tells us if the recipient genuinely read and found value in our content. Understanding how opens are tracked is key to interpreting this metric correctly.
The danger of inflated opens
Relying solely on open rates, especially those artificially boosted by privacy features, can lead to mailing disengaged or even actively hostile recipients. If we only look at opens, we might continue sending emails to users who are simply having their mail pre-fetched, or worse, who opened the email with the intent to report it as spam.
Increased complaints: If recipients are not truly engaged but their actions are being counted as opens, they might eventually mark your emails as spam, leading to higher complaint rates.
Damaged sender reputation: Email Service Providers (ESPs) and Inbox Service Providers (ISPs) track various signals beyond opens to assess sender reputation. If your genuine engagement is low but you keep mailing based on inflated open rates, it can negatively impact your sender score and deliverability.
Wasted resources: Sending emails to unengaged recipients wastes your email sending limits and marketing budget, diverting resources from truly interested subscribers.
Limitations and misinterpretations
A crucial limitation of open rates is that an open does not automatically equate to positive intent. Sometimes, a recipient may open an email not out of interest in the content, but to find a way to unsubscribe, to identify the sender for a block, or even, in rare cases, to gather information to report the sender to legal counsel. These are still counted as opens, yet they represent a negative interaction, completely misleading marketers who rely solely on this metric for engagement.
Continuing to email these negatively engaged recipients, or those whose opens are merely bot-driven, can severely impact your sender reputation. If you don't account for negative signals like spam complaints through feedback loops (FBLs), you risk sending more mail to individuals who don't want it. This inevitably leads to a higher rate of messages landing in the spam folder, a clear sign of deteriorating domain reputation.
Another subtle but dangerous misinterpretation comes from spam traps. While rare, some spam traps are configured to interact with messages, including loading images, which registers as an open. If your list includes these, their 'opens' will falsely inflate your engagement metrics, while simultaneously signaling to blocklists (or blacklists) that you are sending to problematic addresses. Being placed on a blacklist can severely impact your deliverability.
This highlights why it's critical to look beyond just the raw open rate. When we see high open rates despite low engagement, it's a warning sign, not a success metric. We need to integrate complaint data, bounce rates, and, most importantly, explicit clicks into our analysis to understand the true state of our audience engagement and the health of our email program.
Traditional open rate
Definition: The percentage of emails opened out of those delivered.
Measurement: Historically tracked via a 1x1 pixel loaded when the email is opened.
Primary insight: Subject line appeal and email delivery success.
Limitations: Prone to artificial inflation due to MPP and bot activity, doesn't reflect actual content engagement.
Modern engagement metrics
Click-Through Rate (CTR): Percentage of recipients who clicked a link within the email. A strong indicator of content relevance and call-to-action effectiveness.
Click-to-Open Rate (CTOR): Percentage of unique opens that resulted in a click. Measures how engaging the email's content is after opening.
Conversion Rate: Percentage of recipients who completed a desired action (e.g., purchase, signup) after clicking.
Feedback Loops (FBLs): Provide data on spam complaints directly from ISPs, essential for list hygiene.
Complementing opens with other metrics
Given the limitations of open rates, it's essential to adopt a multi-metric approach to truly understand email campaign performance and recipient engagement. Open rates can still play a role, but they should be viewed in conjunction with other, more explicit signals of interest and satisfaction.
The click-through rate (CTR) and the click-to-open rate (CTOR) are far more reliable indicators of genuine engagement. A click unequivocally demonstrates that a recipient not only opened the email but also found the content compelling enough to take action. CTR measures the percentage of delivered emails that resulted in a click, while CTOR assesses the effectiveness of your content in driving clicks among those who opened the email. These metrics provide deeper insights into content relevance and call-to-action effectiveness.
Beyond clicks, other vital metrics include unsubscribe rates, complaint rates (through feedback loops), and conversion rates. Unsubscribe and complaint rates are crucial for maintaining a healthy list and avoiding blocklists, as they directly reflect negative sentiment. Conversion rates, of course, tie your email efforts directly to business outcomes. By combining these metrics, we get a holistic view of campaign success, audience health, and overall deliverability. It's about monitoring the right KPIs to drive genuine marketing impact.
Strategic use of open rates
Even with their limitations, opens still have a strategic place in email marketing analysis. They are particularly useful for A/B testing elements that influence the initial decision to open, such as subject lines, preheader text, and sender names. By comparing open rates for different variations, we can gauge which elements are most effective at capturing attention and prompting the initial interaction, even if that interaction is sometimes automated.
Furthermore, monitoring open rate trends over time, especially when segmented by ISP (Inbox Service Provider), can offer valuable insights into your overall deliverability and sender reputation. A consistent decline in opens from a specific ISP, not attributable to content changes or MPP, might indicate a problem with your sending practices or a spam filtering issue. Tools like Google Postmaster Tools can provide such aggregate data. While they may not reflect true engagement, they can still point to significant changes in how your emails are being processed at the inbox level.
Metric
Description
Primary Insight
Open rate
Percentage of delivered emails recorded as opened.
Subject line appeal, broad deliverability signal (with caveats).
Click-through rate (CTR)
Percentage of delivered emails with at least one click.
Content relevance, call-to-action effectiveness.
Click-to-open rate (CTOR)
Percentage of unique opens resulting in a click.
Email content engagement efficiency.
Conversion rate
Percentage of recipients completing a desired action.
Business impact and ROI.
Complaint rate
Percentage of recipients marking email as spam.
Audience dissatisfaction, sender reputation risk.
Unsubscribe rate
Percentage of recipients opting out of future emails.
List health and relevance over time.
Views from the trenches
Best practices
Always combine open rates with other metrics like clicks and conversions for a complete view of engagement.
Use open rates primarily for A/B testing subject lines and preheader text to optimize initial appeal.
Monitor open rate trends over time, segmented by ISP, to detect shifts in deliverability.
Common pitfalls
Relying solely on open rates as the primary indicator of email campaign success.
Failing to account for artificial inflation from Mail Privacy Protection (MPP) and bot activity.
Continuing to mail contacts based on opens without considering negative signals like spam complaints.
Expert tips
Focus on engagement segments that show multiple opens and clicks across several campaigns to identify truly active subscribers.
Understand that an open can sometimes be a negative signal, indicating a recipient opening to report or gather information for complaints.
Recognize that while Google may not provide direct feedback loops for spam complaints, other ISPs do, and their data is invaluable for list hygiene.
Expert view
Expert from Email Geeks says relying solely on opens without considering feedback loops for spam hits can increase unwanted mail and damage sender reputation over time.
2020-01-28 - Email Geeks
Marketer view
Marketer from Email Geeks says relying on a single metric for success is generally poor statistical practice, noting that FBLs can vary and recipients might junk mail without opening.
2020-01-28 - Email Geeks
A nuanced approach to email metrics
While email open rates remain a widely cited metric, their utility in isolation has diminished significantly in today's privacy-focused email ecosystem. We can no longer afford to rely on them as the sole, or even primary, indicator of true recipient engagement or campaign success. The era of inflated opens demands a more sophisticated approach.
Instead, opens should be integrated into a broader analytical framework. Combine them with more reliable engagement signals like click-through rates, click-to-open rates, and conversion metrics. Use opens for what they still do best: testing subject line appeal and monitoring high-level deliverability trends. By adopting this nuanced perspective, we can ensure our email marketing strategies are built on accurate insights, fostering genuine engagement and protecting our sender reputation.
Apple, the reliability of open rates as a true measure of engagement has come into question. These privacy changes pre-fetch emails and their tracking pixels, artificially inflating reported open rates. This means a significant portion of what appear to be opens are not actual human interactions.
