Open Rate in Email Marketing ​

Open rate is the percentage of delivered emails that were recorded as opened. It's one of the oldest metrics in email marketing metrics and analytics, traditionally used as a first signal of whether a subject line, sender name, or send time got someone's attention. It's still reported in almost every analytics dashboard, but the way it should be interpreted has changed significantly in recent years.

What Open Rate Measures ​

Conceptually, open rate is a simple ratio:

Open Rate = (Emails Opened ÷ Emails Successfully Delivered) × 100

Technically, an "open" is recorded when a tiny, invisible tracking pixel embedded in the email body loads from the recipient's device. When that pixel image is requested, the sending platform logs an open. This mechanism has worked reasonably well for years, but it depends on an assumption that's no longer reliably true: that the pixel only loads when a real person views the email.

Why Apple Mail Privacy Protection Changed Everything ​

Apple Mail Privacy Protection (MPP), introduced as part of iOS and macOS, pre-fetches and caches remote images (including tracking pixels) for emails read in Apple Mail, regardless of whether the recipient ever actually opens the message. This means:

  • An email can be marked "opened" the moment it arrives, before a human ever sees it.
  • Opens can be recorded even for emails a recipient deletes without reading.
  • Because a large share of inboxes use Apple Mail, this affects a meaningful portion of most senders' lists, not just an edge case.

The practical result is that open rate data is now inflated by an unknown and inconsistent amount, and that inflation varies by list depending on how many subscribers use Apple Mail. Two campaigns with identical real engagement can show different open rates simply because their audience mix of email clients differs.

What Open Rate Is Still Useful For ​

Open rate isn't worthless. It's just no longer precise. It still has legitimate uses when read directionally rather than literally:

  1. Relative subject line testing: In an A/B test where two subject lines are sent to similar audience splits at the same time, the automated-open inflation affects both variants roughly equally, so the comparison between them can still be directionally informative even if the absolute numbers aren't trustworthy.
  2. List engagement trend over time: A steady decline in open rate for the same list, measured consistently over months, can still flag a real problem, such as content fatigue, deliverability issues, or a stale list, even if the absolute percentage is inflated.
  3. Spotting delivery problems early: A sudden, sharp drop in open rate (beyond normal noise) is often one of the first visible symptoms of a deliverability issue, such as landing in spam.

Why Open Rate Shouldn't Be a Sole Success Metric ​

Because a meaningful share of recorded opens may never have been seen by a human, open rate should not be treated as:

  • A precise measure of how many people actually read an email
  • A reliable standalone KPI for campaign success
  • A trustworthy metric in isolation, without corroborating signals

Metrics that require an active, deliberate action from the recipient, like click-through rate, are far harder to trigger automatically, which makes them a more trustworthy signal of genuine engagement. A well-rounded view of campaign performance weighs open rate as one input among several, alongside clicks, conversions, and list health, rather than as the headline number.

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