Email Open Rate

Email open rate is the percentage of successfully delivered emails that recipients have opened. It is a standard metric in email marketing, but its usefulness as a standalone indicator has declined substantially since Apple introduced Mail Privacy Protection (MPP) in September 2021 (Apple, 2021).

How It Is Calculated

The formula is: (Unique Opens ÷ Delivered Emails) × 100. "Delivered" refers to emails that reached an inbox, excluding hard and soft bounces. "Unique opens" counts each recipient once, regardless of how many times they open the same message.

Open rate tracking relies on a small invisible pixel embedded in the email body. When the email loads, the pixel fires and the open is recorded. That mechanism is also the source of the metric's main weakness.

Why Open Rate Has Become Unreliable

Apple's Mail Privacy Protection, rolled out with iOS 15 in September 2021, pre-loads all tracking pixels for Apple Mail users on Apple's own servers, regardless of whether the recipient actually reads the message (Apple, 2021). This means every email sent to an Apple Mail inbox registers as "opened," even if it sits unread. Corporate email security software, including Microsoft's Exchange Online Protection, also scans links and can fire pixels automatically.

The combined effect is that a reported open rate today often blends genuine human attention with automated pixel-loading. Treating it as a direct measure of reader interest can lead to decisions built on distorted data.

Open Rate vs. Click Rate and CTOR

Three related metrics are frequently grouped together but measure different things. Understanding where each begins and ends helps you pick the right one for each decision.

• Open rate measures delivered emails that were opened. It is the most exposed to MPP inflation and automated bot activity, because its pixel-based tracking mechanism is precisely what pre-loaders target.
• Click-through rate (CTR) measures delivered emails that generated at least one link click. A click requires an active human decision, which makes CTR considerably less susceptible to automated distortion and a more reliable gauge of content engagement in email marketing.
• Click-to-open rate (CTOR) measures clicks as a share of opens rather than of all delivered emails. It shows how compelling the email body is for those who opened it, but because it uses open rate as its denominator, it inherits the same reliability problem when MPP inflates the denominator.

For newsletter managers who need a defensible engagement signal, click-through rate and direct replies are more trustworthy than open rate alone. Conversions, such as purchases, sign-ups, or downloads, are among the most reliable measures of all, because they require a deliberate human step that pixel pre-loading cannot replicate.

Application Example

Illustrative scenario, not based on real customer data: A B2B newsletter sends 10,000 emails. The email platform reports 4,100 opens, giving a reported open rate of 41%. Reviewing click data reveals 320 actual link clicks, a click-through rate of 3.2%. A deliverability check shows that a large share of the list uses Apple Mail. The open rate looked strong; the click-through rate tells a more precise story about how many recipients actively engaged with the content.

Why It Still Matters

Open rate retains value in two specific situations. In subject-line A/B tests comparing variants sent to the same list under identical conditions, MPP inflation applies equally to both groups, so relative differences can still indicate which subject line prompted more attention. And a sudden, sharp drop in open rate can surface a deliverability problem, such as messages landing in spam, before other metrics register it.

Pair open rate with click-through rate, direct replies, and conversion data for a fuller picture, and weight it accordingly if a significant share of your list uses Apple Mail.