The Email Metrics That Actually Matter Now That Open Rates Are Unreliable

Reading Email Engagement Metrics Without a Marketing Team

Skip open rate. Since 2021, it’s no longer a reliable signal on its own. Instead, watch these four email engagement metrics:

• Click-to-open rate
• Reply rate
• Complaint rate
• Unsubscribe rate

These four email engagement metrics give you a more honest read on what your email marketing is actually earning you, i.e., no marketing team required.

If you’re running campaigns without an in-house team, that answer alone might raise a question: what’s wrong with open rate? The instinct to start there is natural. It’s usually the first number your ESP dashboard shows you, and it worked well enough through the 2010s.

Since 2021, opens have become an unreliable signal. One segment of contacts might always show as “opened” with zero click-throughs. Another never shows an open at all, even though real people are on the other end reading every word. That’s not a glitch in your dashboard. It’s a shift in what “open” even means now, and it’s exactly why the four metrics above matter more than the one you’re used to checking first.

What Apple’s MPP Changed for Email Metrics

In September 2021, Apple rolled out Mail Privacy Protection (MPP) as part of iOS 15, iPadOS 15, and macOS Monterey. It works by pre-loading email content, including tracking pixels, through Apple’s own servers the moment a message lands in the inbox, long before the recipient actually opens it. It registers as an “open” whether or not a human ever looks at the email.

The scale of the impact on email engagement metrics is hard to overstate. Apple Mail now accounts for more than half of all tracked email opens worldwide, according to ongoing email client market-share research from Litmus, and industry analyses commonly put the resulting inflation at 15 to 20 percentage points or more on lists with a meaningful Apple Mail audience. If your dashboard shows a 40% open rate, a real portion of that number may have nothing to do with a human reading your email.

Apple moved first, but it didn’t stay alone for long. Since 2024, Google, Yahoo, and Microsoft have each rolled out stricter bulk-sender requirements. These aren’t exactly privacy features, but enforcement standards that reshape how your email is measured and whether it gets delivered at all. Senders are now expected to have:

• Three technical authentication records in place: SPF, DKIM, and DMARC
• A working one-click unsubscribe link in the email header and body
• A spam complaint rate that stays under 0.3% — best practice is well under 0.1%

Miss any of these, and your emails risk landing in spam or getting throttled, regardless of what your open rate says. Apple’s privacy move and the 2024 enforcement wave work differently. Still, they point to the same conclusion: the inbox providers carrying your email are actively reshaping email engagement metrics and what gets delivered. Small-business senders who haven’t adjusted are flying without accurate instruments.

[If you’ve already run an ISP distribution audit using the tools in The Best Email Deliverability Audit Tools for B2B Senders, this is the next layer: knowing your emails are landing is only half the picture. Knowing whether anyone’s actually engaging with them once they arrive is the other half.]

Which Email Engagement Metrics Matter Now

Click-to-open rate

Your ESP dashboard will show you all the email metrics that matter. The first of these is your click-to-open rate (CTOR), i.e., clicks divided by opens. It’s more useful than open rate alone, but it isn’t fully immune to the same distortion, since it’s built on the same open counts that MPP inflates.

• CTOR tells you what share of the people who “opened” also clicked something
• A healthy CTOR implies real engagement, especially on lists that skew away from Apple Mail
• On an Apple-heavy list, CTOR will read artificially low even when real engagement hasn’t changed — the padded denominator dilutes the number, not the underlying interest
• Read CTOR as directionally useful, not as a precise benchmark, until you know roughly what share of your list reads through Apple Mail

Reply rate

• Replies are the strongest engagement signal you have
• ISPs treat messages that get replies as wanted mail—it’s a strong positive signal for your sender reputation
• Nearly impossible to fake or inflate
• Even a handful of genuine replies strengthens how inbox providers treat your future sends

Complaint rate

• Track this at the individual send level, not just in aggregate
• A single send to the wrong segment can spike your complaint rate on its own
• A spike can affect inbox placement for the sends that follow it
• Check complaint rates per email routinely, not just monthly

Unsubscribe rate

• A rising unsubscribe rate is feedback on content type or send frequency
• Useful early warning before it becomes a deliverability problem
• Contacts who unsubscribe are telling you something directly
• Contacts who don’t unsubscribe but quietly stop engaging are the harder problem—they don’t show up as a complaint, but they still damage your sender reputation over time

(Metrics framework drawn from PMP’s own eBook, Before You Spend Another Dollar on Email Marketing.)

How Do You Define an Engaged Subscriber Today

One of the clearest signals in this new email engagement metrics landscape is simply how recently a contact last engaged. HubSpot measures in terms of sends rather than time: a specific number of sends, depending on the context. For most B2B cadences, that translates to somewhere between 120 and 180 days depending on how often you’re sending.

You don’t need special tooling to track the email metrics that matter. Most ESPs can filter your list by last-click or last-reply date natively, which is enough to build a genuinely engaged segment without adding new software to your stack.

What Should You Still Watch Your Open Rate For

None of this means open rate is worthless. Are email open rates accurate as a metric? No. However, this measurement is still useful for spotting deliverability trends and catching spam-trap issues. For example, a sudden drop or spike is worth investigating even if the absolute number can’t be trusted. What it’s no longer useful for is measuring performance on its own.

The email metrics that matter have changed over time. Inbox provider dominance shifts slowly for years, then changes overnight. In past years, I’ve seen clients suffer real consequences from holding on too tightly to past practices after one of these shifts and ignoring the email metrics that matter. When your sender reputation takes that kind of hit, the fix is either a strategic reset or a full remediation project, neither of which is quick.

Open Rate Isn’t Dead but It’s Been Demoted

Open rate isn’t dead. It’s just moved into a different role. Like the shift search has gone through as AI answers take over more queries, this particular metric has gotten noisier and now needs interpretation — it’s a directional indicator, not a scorecard.

The good news: you have four action-based email engagement metrics that give you a genuinely clear read on the email metrics that matter, pointing straight at the engagement and results that matter.