Which open rate metric, UOR or TOR, is best for email deliverability assessment?
Matthew Whittaker
Co-founder & CTO, Suped
Published 11 Jun 2025
Updated 19 Aug 2025
7 min read
When assessing email deliverability, the choice between Unique Open Rate (UOR) and Total Open Rate (TOR) often comes up. Both metrics provide insights into subscriber engagement, but their utility for gauging inbox placement and sender reputation differs significantly. Understanding these differences is crucial for any email sender aiming to optimize their campaigns and maintain a healthy email program.
The email landscape has evolved, with privacy changes and advanced filtering mechanisms impacting how opens are tracked. This makes a careful examination of UOR and TOR even more important. It's no longer just about seeing how many times an email was opened, but about understanding what those numbers truly represent in the context of deliverability.
This discussion will delve into both metrics, explore their strengths and weaknesses, and ultimately determine which provides a more reliable signal for your email deliverability assessment. We will also touch upon the broader context of email engagement and its impact on how email service providers view your sending practices.
Understanding open rates for deliverability
Email open rates have historically been a go-to metric for judging campaign performance and, to some extent, deliverability. They reflect the initial engagement a subscriber has with an email after it lands in their inbox. A high open rate typically suggests that your subject lines are compelling and that recipients recognize and trust your sending domain.
However, the reliability of open rates as a direct deliverability indicator has diminished over time. This is primarily due to technological advancements, such as Apple Mail Privacy Protection (MPP), which pre-fetches and caches images, making it appear as if an email has been opened even if the recipient hasn't viewed it yet. This can significantly inflate reported open rates.
Despite these challenges, open rates still offer some value. They remain a proxy for initial interest and can help in A/B testing subject lines or sender names. It's essential, however, to interpret them cautiously and in conjunction with other metrics for a holistic view of your email program's health and deliverability performance.
The unique open rate (UOR)
Unique Open Rate (UOR) counts how many individual subscribers open your email at least once. If a subscriber opens an email multiple times, it's still only counted as one unique open. This metric is designed to give you a clearer picture of how many different people are actually interacting with your messages.
For deliverability assessment, UOR is generally considered more useful than TOR. It provides a more realistic view of recipient engagement, as it filters out repeated opens from the same person (or bot). This makes it a better indicator of whether your emails are successfully reaching the inbox and resonating enough for recipients to engage with them initially.
Even with the challenges posed by MPP, a consistent and healthy UOR can still signal good sender reputation with Mailbox Providers. A sudden drop in UOR might indicate a deliverability problem, such as being routed to the spam folder or placed on a blocklist.
UOR: unique open rate
Definition: Counts the number of individual subscribers who open an email at least once.
Pros: Better indicator of audience reach and initial engagement. Less susceptible to multiple opens from a single user.
Cons: Still impacted by Apple MPP and bot activity, leading to inflated numbers. Can be an expensive operation to calculate at scale.
Deliverability Relevance: Provides insight into if emails reach the inbox. Helps diagnose significant drops in reach.
The total open rate (TOR)
Total Open Rate (TOR) counts every instance an email is opened, regardless of whether it's by the same subscriber opening it multiple times. If a recipient opens an email five times, it counts as five opens towards the TOR. This metric can be useful for understanding overall engagement with an email, but it's less accurate for judging unique reach.
For deliverability assessment, TOR is generally less reliable than UOR. The multiple opens by a single user or automated systems can significantly inflate this metric, making it harder to discern actual inbox placement issues. While a very low TOR might still signal a problem, a high TOR doesn't necessarily mean good deliverability if many of those opens are not unique or are bot-generated.
TOR can be more efficient to calculate at scale, as it simply aggregates all open events. However, its performance benefit might not outweigh its reduced accuracy for critical deliverability insights, especially when Mailbox Providers are looking for genuine human engagement signals.
TOR: total open rate
Definition: Counts every time an email is opened, including multiple opens by the same subscriber.
Pros: Can indicate overall engagement with content if a user revisits the email. More performant to track than UOR at scale.
Cons: Highly susceptible to inflation from automated opens, bots, and repeated user actions. Less accurate for unique reach.
Deliverability Relevance: Less reliable for direct deliverability assessment due to inflated counts. Still, extremely low TOR could indicate a blocklist issue.
The limitations of open rates for deliverability
While UOR is generally better than TOR for deliverability assessment, it's crucial to acknowledge the overall limitations of relying solely on open rates. Mailbox Providers increasingly prioritize other engagement signals due to the inaccuracies introduced by features like Apple Mail Privacy Protection, where open tracking can be unreliable. This means that an email might be opened automatically without human interaction.
Automated opens from bots or security scanners also distort both UOR and TOR. These opens do not reflect genuine subscriber interest and can falsely inflate your engagement metrics, masking underlying deliverability problems. If you see very high open rates but low click or conversion rates, bot activity could be a factor.
For a more accurate understanding of your deliverability and overall campaign success, it's best to shift focus towards other, more reliable engagement metrics. These include clicks, conversions, replies, and even list growth or churn rates. These actions require more explicit user intent and are less prone to automated inflation.
While UOR is technically the better metric for assessing deliverability compared to TOR, the reality is that both are becoming less reliable as primary indicators. The industry is moving towards a focus on more explicit engagement signals due to privacy features like Apple MPP and the prevalence of automated opens.
To truly understand your email deliverability, you need to look beyond opens. Focus on a broader set of metrics that reflect genuine user interaction, such as click-through rates, conversions, and even replies. Combine these with direct deliverability testing methods and monitoring your sender reputation through tools that provide insights from Mailbox Providers.
Key email deliverability metrics
Delivery rate: The percentage of emails that successfully reach recipients' inboxes, without bouncing or being deferred. This is the foundational metric for deliverability.
Bounce rate: The percentage of emails that could not be delivered to the recipient's server. High bounce rates can negatively impact sender reputation.
Complaint rate: The percentage of recipients who mark your email as spam. This is a critical indicator of poor sender reputation and engagement.
Click-through rate (CTR): The percentage of recipients who clicked on a link within your email. A strong indicator of content relevance and engagement.
Unsubscribe rate: The percentage of recipients who opt out of your mailing list. While part of a healthy list, sudden spikes can indicate content or frequency issues.
Conversion rate: The percentage of recipients who complete a desired action, such as a purchase or form submission. This is the ultimate measure of ROI.
Views from the trenches
Best practices
Always prioritize explicit engagement metrics like clicks and conversions over open rates for accurate deliverability assessment.
Regularly monitor your bounce and complaint rates, as these are strong signals of sender reputation issues.
Utilize tools like Google Postmaster Tools for direct feedback from Mailbox Providers on your sending performance.
Segment your audience and personalize content to drive more meaningful engagement beyond just opens.
Clean your email list frequently to remove inactive or problematic addresses to maintain good sender health.
Common pitfalls
Over-relying on open rates as the sole indicator of email deliverability, especially with Apple MPP's impact.
Ignoring high bounce rates, which can signal issues with list hygiene and lead to blocklisting.
Failing to track complaint rates, a critical metric that directly impacts your sender reputation.
Not differentiating between human and bot opens, leading to inflated engagement data and misguided optimization efforts.
Neglecting to warm up new IP addresses or domains, which can result in poor deliverability from the start.
Expert tips
Consider how content effectiveness influences conversions, as high deliverability with poor content won't yield results.
Recognize that UOR offers a more nuanced view of deliverability and pre-open experience effectiveness than TOR.
Understand the relationship between TOR and UOR in your data to calibrate your benchmarks for assessing list deliverability.
Be aware that increased filtering, particularly by Gmail, will make cold marketing less effective over time.
Focus on customer-centric metrics like conversions while also recognizing that Mailbox Providers value engagement signals like UOR.
Expert view
Expert from Email Geeks says that testing is crucial to understand the data, highlighting the importance of empirical observation.
2019-12-11 - Email Geeks
Expert view
Expert from Email Geeks points out that conversion rate is the most critical metric, especially when evaluating campaign ROI, and highlights the increasing unreliability of open rates due to email client configurations and URL tracking issues.
2019-12-11 - Email Geeks
Key takeaway: UOR and the evolving landscape
For email deliverability assessment, Unique Open Rate (UOR) offers a more refined metric than Total Open Rate (TOR). UOR helps in understanding how many individual recipients are actually engaging with your emails, providing a less inflated view of engagement compared to TOR, which counts every single open, including automated ones.
However, with the widespread adoption of Apple Mail Privacy Protection and increasing bot activity, relying solely on any open rate metric for deliverability is becoming less accurate. It's more effective to integrate UOR with other strong engagement metrics, like click-through rates and conversions, alongside dedicated deliverability monitoring practices such as seed list testing and analyzing Google Postmaster Tools.
Delivery rate: The percentage of emails that successfully reach recipients' inboxes, without bouncing or being deferred. This is the foundational metric for deliverability.
Bounce rate: The percentage of emails that could not be delivered to the recipient's server. High bounce rates can negatively impact sender reputation.
Complaint rate: The percentage of recipients who mark your email as spam. This is a critical indicator of poor sender reputation and engagement.
