For years, email performance has been measured by a familiar set of metrics: opens, clicks, and conversions.
But these numbers are becoming less useful, not just because of privacy changes, but because mailbox providers (MBPs) now judge emails like users do: based on engagement, trust, and intent.
This shift is forcing marketers to reconsider how they measure success.
A new report from Validity points to three emerging metrics that are likely to become mainstream in 2026 and say more about subscriber relationships than traditional KPIs ever could.
1. The disaffection index signals when the public turns away
The first parameter turns the usual performance mentality on its head.
Instead of focusing on positive engagement, the disaffection index measures signs that an audience is losing interest, combining cancellations, complaints and bounces into a single metric.
In other words, it answers a question that most dashboards ignore: How fast are you burning out your audience?
This matters because inbox providers increasingly treat negative signals as stronger indicators than positive ones. A campaign with decent click-through rates can still compromise delivery if it generates enough complaints or cancellations.
The conclusion for marketers is simple: optimizing for clicks alone isn’t enough. Lasting performance depends on minimizing friction and fatigue over time.
2. Response rate is the true measure of engagement
Clicks can be accidental. Openings are often unreliable. Responses, on the other hand, require intention.
That’s why response rate is emerging as a key signal of engagement. In the report example, even a 1% response rate represents a significant level of investment from the audience – these are subscribers who cared enough to respond.
Mailbox providers increasingly interpret this type of interaction as a strong signal of trust. Emails that invite and generate responses are more likely to be seen as relevant and placed in the inbox accordingly.
This has practical implications for your email strategy. Campaigns are starting to look less like broadcasts and more like conversations, with requests for feedback, questions or direct answers integrated into the content.
It also introduces operational challenges. As the volume of responses grows, more and more teams are turning to automation or AI agents to manage responses at scale, effectively combining email marketing with customer service.
3. Quantify trust
“Building trust” has long been a marketing cliché. What changes is the attempt to measure it.
The report outlines a framework for trustworthiness that combines credibility, reliability and intimacy, tailored to how inwardly focused a brand appears to be.
While subjective, these components closely match how subscribers rate emails:
- Credibility: Does this sender know what they are talking about?
- Reliability: Do they consistently follow up?
- Intimacy: Do I feel safe in relating to them?
- Self-orientation: Is it about me or just about them?
The challenge is to turn them into usable metrics. Suggested approaches include tracking responses, tracking complaint rates, encouraging preference updates, and actively gathering feedback.
What is noteworthy is that trust can move in both directions, and negative signals often carry more weight.
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Switch from campaign metrics to relationship metrics
Taken together, these three metrics reflect a broader shift in email marketing.
Performance is no longer just about what happens immediately after a send. It’s about how recipients perceive and interact with a brand over time.
This is in line with how mailbox providers now filter mailboxes. Engagement is not just measured, but interpreted. And the signals they prioritize increasingly resemble relationship health, not campaign performance.
For marketers, this means that success is becoming increasingly difficult to play.
And that’s probably the point.
Validity Link to benchmark report via email 2026. (Registration required)
