Email Marketing KPIs: The Metrics That Actually Predict Revenue Growth

Most e-commerce brands measure their email programme by open rate. They check it after every send, compare it week to week, and treat it as the primary signal of whether email is “working.” The problem is that open rate in 2025 is an unreliable metric for a large portion of your audience, and even if it were reliable, it would tell you almost nothing about revenue trajectory.

This post builds a proper KPI hierarchy for e-commerce email marketing: the metrics that actually predict whether your programme is growing your business, and the ones you should stop treating as primary indicators.

Why Open Rate Is No Longer a Reliable Primary Metric

In September 2021, Apple introduced Mail Privacy Protection (MPP) as part of iOS 15. MPP pre-loads email content — including the tracking pixel used to register opens — before the user actually opens the email. The result: every email sent to an Apple Mail user on iOS appears as opened in your ESP’s reporting, regardless of whether the subscriber actually opened it.

Apple’s iOS market share among smartphone users sits consistently above 55% in the UK and US. This means that for most e-commerce email lists, the majority of your subscribers are reporting inflated open rates.

Open rates of 45–60% that brands see post-MPP are not real engagement rates. They are a combination of real opens and phantom opens generated by Apple’s pre-loading. You cannot distinguish between the two in your Klaviyo dashboard.

What open rate is still useful for

Open rate retains value as a directional indicator over time for your non-Apple audience, and as a rough benchmark to flag catastrophic deliverability problems (if open rate drops sharply, something has changed). But as a week-to-week performance metric, it’s no longer reliable.

The KPI Hierarchy That Actually Matters

Tier 1: Revenue Metrics

These are the primary metrics. Everything else is context for understanding them.

Revenue per recipient (RPR)

Total email-attributed revenue divided by total recipients for a campaign or flow. This is the single most important commercial metric for your email programme.

RPR normalises for list size changes, send frequency, and list composition. A campaign that generates £5,000 from 50,000 recipients (£0.10 RPR) is performing worse than a campaign that generates £3,000 from 20,000 recipients (£0.15 RPR), even though the absolute revenue is lower.

Benchmark: RPR varies significantly by product price point and brand. Track your own RPR over time and aim for consistent improvement. As a rough reference, Klaviyo’s data suggests top-performing e-commerce brands achieve RPRs of £0.20–£0.50 for promotional campaigns.

Email-attributed revenue (total and channel split)

Track total email revenue monthly as an absolute number and as a percentage of total store revenue. Also track the split between automation revenue (flows) and campaign revenue.

Benchmark: Email should contribute 20–40% of total e-commerce revenue for brands with a mature programme. Automation should contribute 30–50% of total email revenue. If your automation split is under 20%, your flows are underbuilt.

Revenue per subscriber per month

Total email-attributed revenue divided by engaged list size. This tracks the productivity of your list over time. Growing this metric is the goal of almost every email programme improvement — better segmentation, better flows, better content.

Tier 2: Engagement Metrics

Engagement metrics predict future revenue: high engagement today leads to higher purchase rates tomorrow. They also directly affect deliverability — inbox providers use engagement signals to decide where to place your mail.

Click rate

The percentage of total recipients who clicked a link in the email. Unlike open rate, click rate is not distorted by MPP — a click requires a real human action.

Benchmark: 1.5–3.5% click rate is healthy for e-commerce promotional campaigns. Below 1% suggests a content, relevance, or targeting problem. Above 4% is excellent.

Click-to-open rate (CTOR)

Clicks divided by opens. Even though total opens are inflated, the ratio of clicks to opens tells you something about content quality: of the people who opened (some real, some phantom), how many engaged with the content?

Benchmark: 8–15% CTOR is a healthy range for e-commerce. Below 5% suggests weak email content or misaligned offer.

These metrics are not primary indicators:

Open rate (for reasons covered above) and unsubscribe rate (a lagging indicator that tells you something has already gone wrong — it doesn’t predict future performance).

Tier 3: List Health Metrics

List health metrics predict the future capacity of your programme to generate revenue.

List growth rate

Net monthly subscriber growth as a percentage of your total list. A growing list means future revenue potential is expanding. A flat or shrinking list means you’re on a treadmill — churn is eating your acquisitions.

Benchmark: 3–5% net monthly list growth is healthy for a scaling e-commerce brand. If your list growth rate is below 2%, acquisition channel work is needed.

Engaged subscriber rate

The percentage of your total list that has engaged (opened or clicked) in the last 90 days. This metric tells you the effective size of your list — the portion that is actually active and potentially responsive to campaigns.

Benchmark: Above 25% is healthy. Below 15% means a significant portion of your list is dormant and your effective audience is much smaller than your total list size suggests.

List composition trend

The proportion of your list made up of never-purchased subscribers vs one-time buyers vs repeat buyers. As your list matures and your programme improves, the repeat buyer proportion should grow. If it’s shrinking, retention is a problem.

Tier 4: Deliverability Signals

Deliverability metrics don’t predict revenue directly, but they can cause dramatic revenue drops if they deteriorate. Monitor them consistently.

Spam complaint rate

The percentage of recipients who marked your email as spam. This is the metric inbox providers weight most heavily when deciding where to deliver your mail.

Benchmark: Under 0.08% is Google’s published guideline. Above 0.3% will trigger significant filtering. Monitor this in Google Postmaster Tools (for Gmail) and your Klaviyo dashboard.

Hard bounce rate

The percentage of sends that result in a hard bounce (permanent delivery failure — address doesn’t exist). High bounce rates signal list hygiene problems and damage sender reputation.

Benchmark: Under 0.5% per campaign. Above 1% is a warning sign.

Domain reputation (via Google Postmaster Tools)

Google Postmaster Tools provides a domain reputation score (High, Medium, Low, Bad). This is the clearest direct signal of how Gmail is evaluating your sending domain.

Target: High reputation. Medium means improvement is needed. Low or Bad requires immediate remediation work.

Building a Weekly Email Health Scorecard

The most useful application of this KPI framework is a simple weekly scorecard — a one-page view of your programme’s health that takes 15 minutes to update and gives you immediate visibility of any deteriorating metrics.

The scorecard structure:

Revenue summary

• Email-attributed revenue this week vs last week vs same week last year
• RPR for campaigns sent this week
• Automation revenue contribution (% of total email revenue)

Engagement summary

• Click rate for campaigns sent this week (vs 90-day average)
• CTOR for campaigns sent this week (vs 90-day average)

List health summary

• Total engaged subscribers (vs last week)
• Net list growth this week (new subscribers minus unsubscribes)

Deliverability summary

• Spam complaint rate (latest available)
• Domain reputation status (Google Postmaster Tools)

Any metric that falls meaningfully below its trend line gets flagged for investigation. This is how you catch problems early — before a deliverability issue has been running for three months, or before a broken flow has been missing revenue for six weeks.

From Metrics to Decisions

Metrics are only valuable if they drive action. The weekly scorecard should end with one or two questions: what’s performing differently from expectation, and what does that indicate we should investigate or change?

A click rate that drops from 2.8% to 1.6% over three weeks is a signal worth investigating. Is it a content problem? A segmentation issue? A deliverability problem? The metric flags the issue — investigation finds the cause — and action fixes it.

At Excelohunt, we build custom reporting dashboards and KPI frameworks for e-commerce email programmes, translating raw platform data into the actionable metrics that actually predict growth.

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