What email engagement metrics affect inbox delivery and sender reputation?
Matthew Whittaker
Co-founder & CTO, Suped
Published 20 Jul 2025
Updated 16 Aug 2025
7 min read
Email deliverability is a dynamic landscape, heavily influenced by how recipients interact with your emails. Internet Service Providers (ISPs), like Google and Microsoft, continuously assess sender reputation to filter out unwanted mail. This assessment relies significantly on a complex array of engagement metrics.
Understanding these metrics is not just about tracking numbers, it's about interpreting what they signal to the mailbox providers. Positive engagement tells ISPs that your emails are valued, while negative engagement indicates otherwise, potentially leading to your messages being routed to the spam folder or even a blocklist (or blacklist).
There's no single, universally ranked order of importance for these actions, as each ISP has its own proprietary algorithms. However, certain actions consistently hold more weight than others in shaping your sender reputation and influencing inbox placement.
The fundamentals of engagement metrics
Mailbox providers constantly monitor how recipients interact with emails to determine their overall quality and relevance. These interactions serve as direct feedback signals. Positive interactions suggest that the email is desired and valuable to the recipient, while negative interactions signal the opposite.
The fundamental goal of an ISP is to deliver mail that users want to see. Your sender reputation, a score from 0 to 100, is largely built on how consistently you deliver wanted mail. This score is a critical factor in determining whether your emails land in the inbox, the promotions tab, or the dreaded spam folder.
Positive engagement metrics directly contribute to a healthier sender reputation. These are actions that recipients take which signal interest in your content. Conversely, a lack of positive engagement or an increase in negative engagement can quickly degrade your reputation, leading to lower inbox placement rates. Understanding how ISPs track engagement is key to maintaining good standing.
Positive actions
Adds to contacts: When a recipient adds your email address to their address book, it's a very strong signal of trust and desire for your mail.
Opens: Opening an email, while less impactful than a click or reply, still signifies that the recipient wanted to view your message.
Forwards: Forwarding your email to someone else indicates that the content is highly valuable and shareable.
Negative actions
Marks as spam: This is the most damaging signal, directly telling ISPs your mail is unwanted and should be blocked.
Deletes unread: While not as severe as a spam complaint, consistently deleting emails without opening them suggests a lack of interest.
Unsubscribes: Though it reduces your list size, an unsubscribe is better than a spam complaint. It's a clear signal that the user no longer wishes to receive your emails, allowing for list hygiene.
Bounces: Hard bounces, indicating invalid addresses, severely damage your sender reputation. Soft bounces are temporary issues but repeated occurrences can also be detrimental.
Deep dive into key positive signals
Among the positive signals, actions like recipients adding you to their safe senders list or directly replying to your emails are often considered the most impactful. A reply, in particular, often triggers an automatic addition to the recipient's address book, which acts as a powerful trust signal for ISPs. This level of interaction shows explicit desire for your mail.
Clicks are generally valued more highly than opens, as they demonstrate a deeper level of engagement beyond just viewing the email. However, the reliability of open rates as a metric has changed, particularly with the introduction of Apple Mail Privacy Protection (MPP), which pre-fetches and caches email content, leading to inflated or inaccurate open data for users on Apple devices. This makes focusing on email clicks a more reliable indicator of true engagement.
Other positive actions, like scrolling within an email, may be tracked by some ISPs, especially within their webmail interfaces. However, the value assigned to such metrics is less clear, as a recipient might be scrolling to find an unsubscribe link, rather than engaging positively with the content. The challenge lies in the black-box nature of many ISP algorithms, making it difficult to definitively rank every positive action.
ISP tracking nuances
ISPs like Google and Microsoft possess a deeper understanding of user interactions within their own ecosystems, such as webmail interfaces and managed mobile clients. They gather extensive data on how users engage with emails. Conversely, Apple's Mail Privacy Protection significantly limits how open rates are tracked, as it automatically loads remote content. This means relying solely on open rates can be misleading for segments of your audience using Apple devices. Always consider your audience's email clients when interpreting engagement data.
Understanding negative engagement and its impact
While positive engagement builds trust, negative engagement can rapidly destroy it. Spam complaints are the most critical negative signal. Each time a recipient marks your email as spam, it's a direct and severe message to the ISP that your mail is unwanted. High complaint rates can lead to your sending IP or domain being added to a blocklist (or blacklist), resulting in widespread delivery failures. It's crucial to understand what happens when your domain is blocklisted.
Bounce rates also significantly impact your sender reputation. Hard bounces indicate permanent delivery failures due to invalid email addresses and should be immediately removed from your list to prevent further damage. Soft bounces, while temporary, can also hurt your reputation if they occur frequently, signaling issues like full inboxes or server problems. Maintaining good email list quality is paramount.
Long-term inactivity, even without explicit negative actions, can be detrimental. If recipients consistently delete your emails without opening them, or simply ignore them for extended periods, ISPs may classify your mail as greymail. This means it's not spam, but it's not highly valued, leading to reduced inbox placement. According to Attentive's blog on email deliverability, low open, click, and reply rates can lead to emails being blocked or missing the inbox.
Metric
Impact on reputation
Deliverability effect
Spam complaints
Severe negative
Direct routing to spam folder, blocklisting (also known as blacklisting), and IP/domain reputation damage. Learn about email blocklists in detail.
Hard bounces
Moderate to severe negative
Immediate delivery failure, signals poor list hygiene, and can lead to reputation degradation over time.
Long-term inactivity (greymail)
Gradual negative
Increased likelihood of landing in spam or promotions folders, indicating low relevance to the recipient.
Unsubscribes
Neutral to slightly positive
Though it reduces list size, it's a healthy way for recipients to opt out, preventing potential spam complaints.
Beyond the numbers: Context and audience
It's important to recognize that engagement metrics aren't a one-size-fits-all solution. The relevance and impact of these metrics can vary significantly depending on your audience. For example, B2B email deliverability often involves different engagement patterns compared to B2C. In B2B, emails might be forwarded internally or archived without a direct click, yet they are still considered desired and valuable. ISPs may account for these differences when evaluating B2B sender reputation, as discussed in how email engagement affects inbox placement for B2B emails.
Furthermore, there's a delicate balance between sending enough email to generate revenue and maintaining a healthy sender reputation. Sending to perpetually unengaged users, even if it yields some occasional revenue, carries a hidden cost: the erosion of your domain's trustworthiness with ISPs. This degradation can eventually lead to widespread deliverability issues, jeopardizing your entire email program. Prioritizing list hygiene is a cornerstone of boosting email deliverability rates.
The long-term health of your email program should always outweigh short-term gains from disengaged segments. ISPs are increasingly sophisticated in identifying senders who prioritize volume over relevance. To improve email deliverability, a strategic approach involves proactive list management, monitoring diverse engagement metrics, and adapting sending practices based on what your audience actually values. This proactive stance helps prevent the deliverability consequences of decreased metrics.
Views from the trenches
Best practices
Continuously monitor diverse engagement metrics using tools like Postmaster pages to identify trends and potential issues early on, rather than waiting for problems to become severe.
Implement a clear re-engagement strategy for inactive subscribers that encourages them to confirm their interest or gracefully opt-out, preserving your sender reputation.
Segment your email list based on engagement levels, tailoring content and frequency to ensure that active subscribers receive the most relevant emails.
Common pitfalls
Relying solely on immediate revenue gains from unengaged email segments without considering the long-term degradation of sender reputation and overall deliverability.
Ignoring subtle declines in engagement metrics, which can be early indicators of a worsening sender reputation or approaching deliverability issues.
Applying a one-size-fits-all engagement strategy across all audience types, neglecting the distinct behaviors of B2B versus B2C recipients.
Expert tips
Proactively removing perpetually unengaged subscribers, even if they occasionally generate revenue, is crucial for long-term email program health and consistent inbox placement.
Understand that what you can measure internally is often vastly different from the proprietary metrics and algorithms ISPs use to assess sender reputation.
Focus on proving the negative long-term impact of sending to unengaged users through data modeling, rather than just arguing against it based on perceived best practices.
Expert view
Expert from Email Geeks says that mailbox providers determine engagement with a complex set of metrics, all designed to identify mail that their recipients truly want.
2023-06-19 - Email Geeks
Expert view
Expert from Email Geeks says that different ISPs employ varied factors in their measurement of engagement, meaning there's no single universal ranking of importance for all actions.
2023-06-20 - Email Geeks
Optimizing for a healthy email ecosystem
Ultimately, email engagement metrics are the pulse of your sender reputation and a direct influencer of your inbox delivery. Positive interactions like replies and clicks validate your sending practices, while negative signals like spam complaints and bounces indicate areas for immediate improvement. The interplay of these metrics forms the foundation of how ISPs perceive your email program's trustworthiness.
Given the nuanced and often opaque nature of ISP algorithms, continuous monitoring of your key performance indicators (KPIs) is essential. Regularly assess your email deliverability trends and adjust your sending strategies to align with what your audience genuinely wants. This includes proactive list hygiene and targeted content delivery.
By prioritizing sending relevant, wanted emails to an engaged audience, you not only improve your inbox placement but also build a sustainable and healthy sender reputation that yields long-term success.
Google and 
Microsoft, continuously assess sender reputation to filter out unwanted mail. This assessment relies significantly on a complex array of engagement metrics.
Google and 
Microsoft possess a deeper understanding of user interactions within their own ecosystems, such as webmail interfaces and managed mobile clients. They gather extensive data on how users engage with emails. Conversely, Apple's Mail Privacy Protection significantly limits how open rates are tracked, as it automatically loads remote content. This means relying solely on open rates can be misleading for segments of your audience using Apple devices. Always consider your audience's email clients when interpreting engagement data.
