For most of email marketing’s history, open rate was the headline number. You’d send a campaign, check your dashboard the next morning, and the first thing you’d look at was how many people opened it. It felt like a clean, simple signal of whether your subject line worked and whether your audience cared.
Then Apple released iOS 15 in September 2021, and that signal got a lot noisier.
With Mail Privacy Protection (MPP), Apple started pre-loading email content, including tracking pixels, for subscribers using Apple Mail with privacy protection enabled. The result: opens get recorded whether or not a real person actually opened your email. For senders with a significant Apple Mail audience (which, given Apple’s market share, is most of us), open rates inflated overnight and stopped reflecting reality.
I remember watching open rates jump noticeably in the weeks after MPP rolled out. On the surface, it looked great. In practice, the number had become unreliable. So what should you be looking at instead?
Why Open Rates Were Already Unreliable
Even before Apple changed the game, open rates had structural limitations worth understanding.
The mechanics: your ESP embeds a tiny invisible image (a tracking pixel) in each email. When that image loads, an open gets recorded. The problem is that image loading and actually reading an email are not the same thing.
Several factors distort the count:
- Image blocking: Many email clients block images by default. If the pixel doesn’t load, the open doesn’t register, even if the subscriber read every word.
- Preview panes: Some clients display a snippet of the email in a preview pane, which can trigger a pixel load without the subscriber ever opening the email properly.
- Multiple opens: A single subscriber opening the same email several times counts as multiple opens (or one unique open, depending on your ESP’s reporting). Worth knowing when you’re reading the data.
- Spam filters: Some filtering systems pre-fetch email content, including pixels, to check for malicious content. That can register as an open from a bot, not a human.
- Privacy-focused clients: Hey.com actively strips tracking pixels before they reach subscribers. You’ll get zero opens recorded from Hey users, regardless of how engaged they actually are.
Put all of this together and you have a metric that consistently over-counts in some places and under-counts in others. Using it as your primary success indicator means making decisions on shaky ground.
The Metrics Worth Tracking Instead
None of this means open rates are completely worthless. They still give you a rough directional signal, particularly for subject line testing within a consistent send environment. But they should not be your headline metric.
Here’s what I’d look at instead.
Click rate is probably the most straightforward replacement. If someone clicked, they engaged enough to act. That’s a real signal, not a pixel firing. Track it at the campaign level and by individual link to understand which content is actually driving action.
Click-to-open rate (CTOR) goes one level deeper. It measures clicks as a percentage of opens rather than total recipients, which isolates the quality of your content and offer from your subject line performance. A high CTOR on a low open rate tells you your content resonates once people are in, but your subject line isn’t pulling them there. These are two different problems that need two different fixes.
Conversion rate is where the real story is. Clicks are good. Conversions are what you’re actually after. Whether that’s a purchase, a sign-up, a download, or a booked demo, tracking what people do after they click is what connects your email program to real business outcomes.
Revenue per email takes conversion one step further and is particularly useful for e-commerce senders. It lets you compare campaigns on equal footing regardless of list size or send frequency.
Unsubscribe rate is an underrated signal. A spike in unsubscribes tells you something went wrong: the content missed, the frequency was too high, or the segment wasn’t right for that send. Watch it per campaign rather than as a rolling aggregate, so problems show up quickly.
Spam complaint rate is the one you really cannot afford to ignore. Most ESPs don’t surface this prominently, but it matters enormously for deliverability. Google and Yahoo’s 2024 sender requirements made this explicit: a complaint rate above 0.10% starts affecting inbox placement, and above 0.30% is serious territory. If you’re not actively monitoring this, you may not know you have a problem until your deliverability has already taken a hit.
Bounce rate, particularly hard bounces, is worth watching closely. Sending to a list with more than 2% hard bounces signals to mailbox providers that your list hygiene is poor. Above 5% and you’re likely looking at deliverability consequences that take time to recover from.
Making Sense of Your Data
Tracking more metrics only helps if you actually do something with them.
Set baselines before you start optimizing. Your industry average click rate is interesting context, but what matters more is your own historical performance. Know what normal looks like for your list before you try to move it.
Compare campaigns against each other, not just against benchmarks. If your last ten campaigns averaged a 2.1% click rate and your latest came in at 1.3%, that’s worth investigating, even if 1.3% sits above the industry average.
Segment your analysis. A campaign to your most engaged subscribers will look very different from a re-engagement send. Lumping them together obscures what’s actually happening with each audience.
And test one variable at a time. Subject line, send time, content: all worth testing. Running three experiments simultaneously makes it impossible to know what actually drove the result.
Open rates are not going away, and they are not entirely useless. But treating them as the primary measure of your email program’s health is increasingly hard to justify, particularly post-MPP.
The metrics that replaced them are more actionable anyway. Click rate, CTOR, conversion rate, and spam complaint rate all tell you something you can act on. Open rate, at this point, mostly tells you whether Apple’s servers decided to load a pixel.
If you’re rethinking how you measure performance, it’s worth looking at what’s inside your emails too. Alterable lets you add dynamic, real-time content to your campaigns, which tends to have a measurable effect on the metrics that actually matter.
Alterable helps email marketers add real-time personalized content to their campaigns — countdown timers, dynamic products, location-based images, and more.
